Chapter 4. Tools

Dart provides several tools to help you write and deploy your web and command-line apps. These tools include:

pub: The Dart package manager

Download and install packages of libraries.

Dart Editor

Edit, run, and debug web and command-line apps.

Dartium: Chromium with the Dart VM

Run Dart web apps. This is a special build of Chromium (the project behind Google Chrome).

dart2js: The Dart-to-JavaScript compiler

Convert your web app to JavaScript, so it can run in non-Dartium browsers.

dart: The standalone Dart VM

Run your command-line apps—server-side scripts, programs, servers, and any other apps that don’t run in a browser.

dartanalyzer: The Static Analyzer

Analyze your Dart source from the command line.

All of these tools are in the Dart Editor bundle, since the editor uses them. You can also download Dartium separately, and you can download an SDK that includes pub, dart2js, dart, dartanalyzer, and more. See the Tools page for links and details.

The tools are in the dart-sdk/bin directory of your Dart installation directory. If you intend to use tools such as pub, dart2js, and dart from the command line, add the bin directory to your path.

pub: The Dart Package Manager

You can use the pub tool to manage Dart packages. A Dart package is simply a directory containing any number of Dart libraries and a list of the library dependencies. A package can also contain resources for its libraries, such as documentation, tests, and images. If your app uses one or more packages, then your app itself must be a package.


Dart Editor offers support for using pub, including creating, installing, updating, and publishing packages.

A package can live anywhere. For example, some packages are on GitHub. The Dart team publishes packages at, and we hope you will, too.

To use a library that’s in a Dart package, you need to do the following:

  1. Create a pubspec (a file that lists package dependencies and includes other metadata, such as a name for your package).

  2. Use pub to get your package’s dependencies.

  3. Import the library.

Creating a Pubspec

To use a package, your application must define a pubspec that lists dependencies and their download locations. The pubspec is a file named pubspec.yaml, and it must be in the top directory of your application.

Here is an example of a pubspec that specifies the locations of two packages. First, it points to the js package that’s hosted on, and then it points to the intl package in the Dart SDK:

name: my_app
  js: any
  intl: any

For details, see the pubspec documentation and the documentation for the packages you’re interested in using.

Installing Packages

Once you have a pubspec, you can run pub get from the top directory of your application:

cd path/to/my_app
pub get

This command determines which packages your app depends on, and puts them in a central cache. For git dependencies, pub clones the git repository. For hosted dependencies, pub downloads the package from Transitive dependencies are included, too. For example, if the js package depends on the unittest package, the pub tool grabs both the js package and the unittest package.

Finally, pub creates a packages directory (under your app’s top directory) that has links to the packages that your app depends on.

Importing Libraries from Packages

To import libraries found in packages, use the package: prefix:

import 'package:js/js.dart' as js;
import 'package:intl/intl.dart';

The Dart runtime takes everything after package: and looks it up within the packages directory for your app.

More Information

Pub has support for building web apps. If your web app’s directory structure follows pub’s package layout conventions, you can use the pub development server (pub serve) to continuously build and serve the app’s assets. Once you’re ready to deploy the web app, use pub build to generate the final files.

For more information about pub, see the pub documentation.

Dart Editor

We already introduced Dart Editor in Up and Running. Here are some more tips on using Dart Editor, with information such as specifying a browser and compiling to JavaScript. If you run into a problem, see Troubleshooting Dart Editor. Dart Editor is updated frequently, so it probably looks different from what you see here. For the latest information, see the Dart Editor documentation.

Viewing Samples

The Welcome page of Dart Editor (Figure 1-2) displays a few samples. To open a sample and look at its source code, click the sample’s image.

If you don’t see the Welcome page, you probably closed it. Get it back with Tools→Welcome Page.

Managing the Files View

The Files view shows the files that implement the libraries included in Dart, installed packages, as well as all the apps that you create or open.

Adding apps

Here’s how to open an app, which makes it appear in your Files view:

  1. Go to the File menu, and choose Open Existing Folder.... Or use the keyboard shortcut (Ctrl+O or, on Mac, Command-O).

  2. Browse to the directory that contains the app’s files.

  3. Click Open.

The directory and all its files appear in the Files view.

Removing apps

You can remove an app from the Files view, either with or without deleting its files.

Right-click (or Control-click) the directory and choose Delete. If you want to delete the app’s files permanently, then in the dialog that comes up, choose Delete project contents on disk.

Creating Apps

It’s easy to create a simple web or command-line app from scratch:

  1. Click the New Application button (at the upper-left of Dart Editor). Alternatively, choose File→New Application from the Dart Editor menu. A dialog appears.

  2. Type in a name for your application—for example, hello_web. If you don’t like the default parent directory, type in a new location or browse to choose the location.

  3. Unless you really don’t want files to be automatically created for you, make sure Generate sample content is selected.

  4. Select the kind of application you’re writing, such as command line, web, or web using Polymer.dart.

  5. Click Finish to create a directory with initial files for the app.

    A default Dart file appears in the Edit view, and its directory appears in the Files view. Your Dart Editor window should look something like Figure 4-1.

    A new app, pre-filled with basic, runnable code
    Figure 4-1. A new app, pre-filled with basic, runnable code

Editing Apps

Dart Editor provides the basic editing functionality you’d expect, plus features such as Dart code completion, API browsing, support for refactoring, and the ability to search multiple files.

Using autocomplete

Autocomplete suggestions look something like Figure 4-2. They appear when you either:

  • Type a class or variable name, and then type a period.

    For example, type document. or DateTime. and pause a moment. Once the suggestions appear, continue typing to pare down the list.

  • Press Ctrl+Space.

    For example, type Dat, then Ctrl+Space to see a list of classes that start with “Dat”.

When the suggestions come up, you can click, type, or use the arrow keys to select the one you want. Press Enter or Return to choose a suggestion, or Esc to dismiss the panel.

Autocomplete suggestions
Figure 4-2. Autocomplete suggestions

Browsing APIs

With Dart Editor you can easily find where APIs are declared. You can also outline the contents of a Dart file.

Finding out more about an API

To get more information about an API item—variable, method, type, library, and so on—hover the mouse over the item in the Edit view. Dart Editor displays a popup that provides more information about that item.

To go to the declaration of an API item, either within the same .dart file or in another file:

  1. In the Edit view of a Dart file, click on an API item. Dart Editor highlights the item and all other occurrences of the item.

  2. Right-click and choose Open Declaration from the menu.

    The editor displays the file that declares the item. For example, if you open the declaration for querySelector, the file that declares the querySelector() function appears.

Outlining a file’s contents

Press Alt+O (Option-O on Mac) or right-click and choose Outline File.

A panel comes up displaying the classes, methods, and fields declared in the current Dart file. For example, the outline for the Sunflower sample’s sunflower.dart file looks something like Figure 4-3.

The Outline panel for the Sunflower sample
Figure 4-3. The Outline panel for the Sunflower sample

You can reduce the size of the list by typing one or more characters. For example, if you type c, only the center and context variables appear.

If you choose an item from the list—for example, centerX—the editor scrolls to its declaration.

Alternatively, add a more permanent outline view by choosing Tools→Outline.


To change the name of an item throughout your code, put the cursor within (or double-click) the item’s name in the Edit view, and right-click and choose Rename....

You can rename almost anything—local variables, function parameters, fields, methods, types, top-level functions, library prefixes, top-level compilation units, and more. An example of renaming a top-level compilation unit is changing the name of a file that’s sourced by a library.


The search field at the upper right of the Dart Editor window is an easy way to go directly to:

  • Types

  • Files

  • Text inside of files

The scope of a text search is every file in your Files view. Results for text searches come up in a Search view. Within that view, double-click a file to see it in the Edit view. All occurrences of the search string in the Edit view are highlighted.

Running Apps

To run any Dart app, click Dart Editor’s Run button while a file in that app is selected. If you’re working on a web app, Dart Editor brings up a browser window that displays the app’s HTML page, with the app’s code running inside it.

When you run a web application using Dart Editor, by default the app uses the copy of Dartium that’s included in the Dart Editor download, with the Dart code executing directly in the browser. If your launch configuration specifies a browser, then Dart Editor uses dart2js to compile the Dart code to JavaScript that executes in the browser.

Specifying launch configurations

Use Run→Manage Launches to specify as many launch configurations as you like.

For web apps, you can specify the following:

  • HTML file or URL to open

  • Arguments to pass to the browser; for example, --allow-file-access-from-files

  • Checked mode (Dartium only)

  • Enable experimental features (Dartium only)

  • Whether to show the browser’s stdout and stderr output (Dartium only; useful for diagnosing Dartium crashes)

For example, a web app might have a launch configuration for Dartium and several more configurations corresponding to additional browsers you want to test.

You can specify the following for command-line apps:

  • .dart file to execute

  • Working directory

  • Arguments to pass to the app

  • Checked mode

Running in production mode

By default, apps run in checked mode. To run in production mode instead, disable checked mode in your app’s launch configuration:

  1. Run your app at least once, so that it has a launch configuration.

  2. Choose Run→Manage Launches, or click the little arrow to the right of the Run button and choose Manage Launches.

  3. In the Manage Launches dialog, find a launch configuration for your app. Click it if it isn’t already selected.

  4. Unselect Run in checked mode (see Figure 4-4).

    To run in production mode, unselect checked mode
    Figure 4-4. To run in production mode, unselect checked mode
  5. Click Apply to save your change, or Run to save it and run your app.

For details about checked mode and production mode, see Runtime Modes.

Specifying a browser for JavaScript launches

To specify the browser in which to run the JavaScript version of your web apps:

  1. Choose Dart Editor→Preferences.

  2. Click Run and Debug.

  3. Unless you want to use the default system browser, deselect Use default system browser and specify the location of the browser you want to use. For example, /Applications/

  4. Click OK to save your changes.

Now whenever you run an app as JavaScript, Dart Editor invokes your browser of choice.

Alternatively, you can run your app as JavaScript using the default browser setting and then copy and paste the URL into a different browser.

Debugging Apps

You can debug both command-line and web apps with Dart Editor. Here are some tips:

  • Set breakpoints by double-clicking in the left margin of the Edit view.

  • Use the Debugger view to view your app’s state and control its execution. By default, the Debugger view is to the right of the Edit view and appears when you first hit a breakpoint (Figure 4-5).

  • To see the value of a variable, mouse over it, look in the Debugger view, or use an Expression .

  • To debug web apps, use Dart Editor with Dartium. While you’re debugging, Dart Editor takes the place of the Dartium console. For example, Dart Editor displays the output of print() statements.

Using Dart Editor to debug the Sunflower sample app
Figure 4-5. Using Dart Editor to debug the Sunflower sample app

Compiling to JavaScript

You might not need to do anything to compile your code to JavaScript. When you run an app using a launch configuration that specifies a browser, Dart Editor automatically compiles the app to JavaScript before executing it in the browser.

However, you can also compile Dart code to JavaScript without running the app. Just choose Tools→Generate JavaScript. Another option is using dart2js from the command line (see dart2js: The Dart-to-JavaScript Compiler).

Other Features

Dart Editor has many additional features, including customization and keyboard alternatives.

Customizing Dart Editor

You can customize the editor’s font, margins, key bindings, and more using the Preferences dialog. To bring up the dialog, choose Tools→Preferences (on Mac: Dart Editor→Preferences).

You can also customize which views you see in Dart Editor, as well as their size and position. To add views, use the Tools menu. To remove a view, click its X. To move a view, drag its tab to a different position, either within or outside of the Dart Editor window. To resize a view, drag its edges.

Keyboard alternatives

To get a list of all keyboard alternatives, choose Help→Key Assist (Figure 4-6).

Help→Key Assist
Figure 4-6. Help→Key Assist

Dartium: Chromium with the Dart VM

This section tells you how to get and use Dartium, a Chromium-based browser that includes the Dart VM. This browser can execute Dart web apps directly, so you don’t have to compile your code to JavaScript until you’re ready to test on other browsers.


This browser is a technical preview, and it might have security and stability issues. Do not use Dartium as your primary browser, and do not distribute Dartium to users!

Downloading and Installing the Browser

If you have an up-to-date version of Dart Editor, you already have Dartium.

If you don’t have Dart Editor or want a different version of Dartium, you can get it separately from the Dartium page.

You don’t usually need to do anything special to install Dartium: just unarchive the ZIP file. If you want Dart Editor to launch a particular copy of Dartium, then put that copy in the same directory as the dart-sdk directory of your Dart installation directory (see Step 1: Download and Install the Software), replacing the original copy of Chromium.


The Dartium binary expires after a couple of months. When that happens, you’ll need to download a new binary unless you’re using the Dartium binary that comes with Dart Editor. (Dart Editor automatically updates its Dartium binary.)

Launching the Browser

To launch Dartium, navigate to its directory in your finder, and double-click the Chromium executable file. Or use Dart Editor as described in Running Apps or the command line as described in Launching from the Command Line.


If you already use Chromium: If another version of Chromium is open, then you could have a profile conflict. To avoid this, you can open Dartium or Chromium from the command line with the --user-data-dir flag.

Filing Bugs

If you find a bug in Dartium, create an issue in the Dart project and use the Dartium bug template.

Linking to Dart Source

Use a script tag with a type application/dart to link to your Dart source file. For example:

<!DOCTYPE html>
    <script type="application/dart" src="app.dart"></script>

    <!-- Support for non-Dart browsers. -->
    <script src="packages/browser/dart.js"></script>


Dart Editor automatically adds both the application/dart script tag and the dart.js script tag into the project’s main HTML file.

For more information on linking to source code, see the article “Embedding Dart in HTML.”

Detecting Dart Support

You can check whether a browser supports Dart with this JavaScript code:

// In JavaScript code:
if (navigator.userAgent.indexOf('(Dart)') == -1) {
  // No native Dart support...
  window.addEventListener("DOMContentLoaded", function (e) {
      // ...Fall back to compiled JS...
  }, false);

Launching from the Command Line

Because Dartium is based on Chromium, all Chromium flags should work. In some cases, you might want to specify Dart-specific flags so that you can tweak the embedded Dart VM’s behavior. For example, while developing your web app, you might want the VM to verify type annotations and check assertions. To achieve that, you can enable checked mode (the VM’s --checked flag).

On Linux, you can specify flags by starting Dartium as follows:

DART_FLAGS='--checked' path/chrome

On Mac:

DART_FLAGS='--checked'              \

Or (also on Mac):

DART_FLAGS='--checked'              \
   open path/


You can see the command-line flags and executable path of any Chromium-based browser by going to chrome://version.

dart2js: The Dart-to-JavaScript Compiler

Use the dart2js tool to compile Dart code to JavaScript. Dart Editor uses dart2js behind the scenes whenever Dart Editor compiles to JavaScript. The pub tool’s serve and build options also use dart2js.

The dart2js tool provides hints for improving your Dart code and removing unused code. You can get these hints for all kinds of code—even command-line apps. Also see dartanalyzer, which performs a similar analysis but, as of 1.0, has a different implementation.

This section tells you how to use dart2js on the command line. It also give tips on debugging the JavaScript that dart2js generates.

Basic Usage

Here’s an example of compiling a Dart file to JavaScript:

dart2js --out=test.js test.dart

This command produces a file that contains the JavaScript equivalent of your Dart code. It also produces a source map, which can help you debug the JavaScript version of the app more easily.


Common command-line options for dart2js include:

-o <file> or --out=<file>

Generate the output into <file>; if not specified, the output goes in a file named out.js

-c or --checked

Insert runtime type checks, and enable assertions (checked mode)

-m or --minify

Generate minified output

-h or --help

Display help (use -vh for information about all options)

Some other handy options include:

-p<path> or --package-root=<path>

Specify where to find “package:” imports


Define an environment variable


Display version information for dart2js

The following options help you control the output of dart2js:


Don’t display warnings


Don’t display hints


Emit diagnostics, but don’t suggest how to get rid of the diagnosed problems

-v or --verbose

Display lots of information

The following options control the analysis that dart2js performs on Dart code:


Analyze even the code that isn’t reachable from main(); useful for finding errors in libraries, but using it can result in bigger and slower output


Analyze the code, but don’t generate code


Similar to --analyze-only, but skip analysis of method bodies and field initializers


Use with --analyze-only to analyze a command-line app; default category is Client, which tells dart2js to expect a web app

Helping dart2js Generate Better Code

You can do a couple of things to improve the code that dart2js generates:

  • Write your code in a way that makes type inference easier.

  • Once you’re ready to deploy your app, use the dart2js --minify option to reduce code size.


Don’t worry about the size of your app’s included libraries. The dart2js tool performs tree shaking to omit unused classes, functions, methods, and so on. Just import the libraries you need, and let dart2js get rid of what you don’t need.

Follow these practices to help dart2js do better type inference, so it can generate smaller and faster JavaScript code:

  • Avoid using the dart:mirrors library, directly or indirectly. If you must use it, provide @MirrorsUsed annotations.

  • Don’t use Function.apply().

  • Don’t override noSuchMethod().

  • Avoid setting variables to null.

  • Be consistent with the types of arguments you pass into each function or method.


This section gives tips for debugging dart2js-produced code in Chrome, Firefox, and Safari. Debugging the JavaScript produced by dart2js is easiest in browsers such as Chrome that support source maps.

Whichever browser you use, you should enable pausing on at least uncaught exceptions, and perhaps on all exceptions. For frameworks such as dart:isolate and dart:async that wrap user code in try-catch, we recommend pausing on all exceptions.


To debug in Chrome:

  1. Open the Developer Tools window, as described in the Chrome DevTools documentation.

  2. Turn on source maps, as described in the video SourceMaps in Chrome.

  3. Enable debugging, either on all exceptions or only on uncaught exceptions, as described in Pause on JavaScript exceptions.

  4. Reload your application.


Firefox doesn’t yet support source maps (see bug #771597). To debug in Firefox:

  1. Enable the Developer Toolbar, as described in Kevin Dangoor’s blog post, “New Firefox Command Line Helps You Develop Faster.”

  2. Click the Debugger button at the bottom of the browser, and enable Pause on exceptions (see Figure 4-7).

  3. Reload your application.

Firefox’s Developer Toolbar
Figure 4-7. Firefox’s Developer Toolbar


To debug in Safari:

  1. Turn on the Develop menu, as described in the Safari Web Inspector Guide.

  2. Enable breaks, either on all exceptions or only on uncaught exceptions. See Figure 4-2 on the Safari Debugging page.

  3. Reload your application.

dart: The Standalone VM

You can use the dart tool (bin/dart) to run Dart command-line apps such as server-side scripts, programs, and servers. During development, you also have the option to run command-line apps using Dart Editor.

Basic Usage

Here’s an example of running a Dart file on the command line:

dart test.dart


Common command-line options for dart include:

-c or --checked

Enable both assertions and type checks (checked mode).

-p<path> or -package-root=<path>

Specify where to find imported libraries.


Display VM version information.

-h or --help

Display help. (Add -v for information about all options.)

You can also generate snapshots:


Generate a snapshot in the specified file. For information on generating and running snapshots, see the article Snapshots in Dart.

Enabling Checked Mode

Dart programs run in one of two modes: checked or production. By default, the Dart VM runs in production mode. We recommend that you enable checked mode for development and testing.

In checked mode, assignments are dynamically checked, and certain violations of the type system raise exceptions at runtime. In production mode, static type annotations have no effect.

Assert statements are also enabled in checked mode. An assert statement checks a boolean condition, raising an exception if the condition is false. Assertions do not run in production mode.

You can run the VM in checked mode with the --checked command-line flag:

dart --checked test.dart

dartanalyzer: The Static Analyzer

You can use the dartanalyzer tool (bin/dartanalyzer) to statically analyze your code at the command line, checking for errors and warnings that are specified in the Dart Language Specification.


Dart Editor performs the same analysis that dartanalyzer does. A previous static analyzer, called dart_analyzer, is no longer supported.

Basic Usage

Here’s an example of testing a Dart file:

dartanalyzer --package-root=code/packages test.dart

As Table 4-1 shows, the exit code of dartanalyzer tells you whether the code passed analysis.

Table 4-1. Exit codes for dartanalyzer
Exit codeDescription
0No issues found
1Warnings found (but no errors)
2Errors found


You can use the following command-line options with dartanalyzer:


Specify the directory that contains the Dart SDK

-p<path> or --package-root=<path>

Specify the directory to search for any libraries that are imported using package:


Show warnings not only for code in the specified .dart file and others in its library, but also for libraries imported with package:


Produce output in a format suitable for parsing


Don’t show hints for improving the code


Show the analyzer version

-h or --help

Show all the command-line options


This chapter covered the most commonly used Dart tools. All of them are available in the Dart Editor download, but you can also download Dartium or the SDK separately.

Get Dart: Up and Running now with the O’Reilly learning platform.

O’Reilly members experience books, live events, courses curated by job role, and more from O’Reilly and nearly 200 top publishers.