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<!DOCTYPE html>
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<title>Dart 2 migration guide | Dart</title>
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<h1>Dart 2 migration guide</h1>
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<ul class="section-nav">
<li class="toc-entry toc-h2"><a href="#differences">Differences</a>
<ul>
<li class="toc-entry toc-h3"><a href="#language-and-libraries">Language and libraries</a></li>
<li class="toc-entry toc-h3"><a href="#tools">Tools</a></li>
</ul>
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<li class="toc-entry toc-h2"><a href="#migration">Migrating your code</a>
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<p>Dart 2 has a few key differences from earlier versions of Dart.
This page briefly covers those differences and
gives general advice on migrating your code to Dart 2.</p>
<p>For information on <em>why</em> Dart 2 has changed, see the
<a href="https://medium.com/dartlang/announcing-dart-2-80ba01f43b6">Dart 2 announcement.</a></p>
<h2 id="differences">
<a id="differences" class="anchor" href="#differences" aria-hidden="true"><span class="octicon octicon-link"></span></a>Differences</h2>
<p>The Dart language, libraries, build system, and web development tools have changed.</p>
<h3 id="language-and-libraries">
<a id="language-and-libraries" class="anchor" href="#language-and-libraries" aria-hidden="true"><span class="octicon octicon-link"></span></a>Language and libraries</h3>
<ul>
<li>
<a href="/guides/language/sound-dart">Dart’s type system</a> is now sound.
<ul>
<li><a href="/guides/language/sound-problems">Fixing common type problems</a></li>
<li><a href="https://groups.google.com/d/msg/flutter-dev/H8dDhWg_c8I/_Ql78q_6AgAJ">Flutter announcement: Breaking Change: <code class="highlighter-rouge">--preview-dart-2</code> turned on by default</a></li>
</ul>
</li>
<li>Instance creation keywords are now generally optional,
as described in <a href="/guides/language/language-tour#using-constructors">Using constructors</a>:
<ul>
<li>
<code class="highlighter-rouge">new</code> is always optional.</li>
<li>
<code class="highlighter-rouge">const</code> is optional inside of a constant context.</li>
</ul>
</li>
<li>Dart no longer has checked mode.
<ul>
<li>
<a href="/guides/language/language-tour#assert">Assert statements</a> are still supported, but you enable them differently.</li>
</ul>
</li>
<li>The Dart language and core libraries have changed,
partly as a result of the type system changes.
<ul>
<li><a href="https://api.dart.dev/dev">Dev channel API reference documentation</a></li>
<li><a href="https://github.com/dart-lang/sdk/blob/master/CHANGELOG.md#200">dart-lang/sdk CHANGELOG</a></li>
</ul>
</li>
</ul>
<h3 id="tools">
<a id="tools" class="anchor" href="#tools" aria-hidden="true"><span class="octicon octicon-link"></span></a>Tools</h3>
<ul>
<li>Pub no longer supports transformers.
Instead, use the <a href="https://github.com/dart-lang/build/tree/master/docs">new build system.</a>
</li>
<li>Tools related to web development have changed.
<ul>
<li>The new build system <a href="/tools/build_runner">replaces <code class="highlighter-rouge">pub build</code> and <code class="highlighter-rouge">pub serve</code>.</a>
</li>
<li>Dartium is no longer supported. Instead, use <a href="/tools/dartdevc">dartdevc</a> and Chrome.</li>
</ul>
</li>
</ul>
<h2 id="migration">
<a id="migration" class="anchor" href="#migration" aria-hidden="true"><span class="octicon octicon-link"></span></a>Migrating your code</h2>
<p>How to migrate your code depends on how old your code is
and what platforms it runs on.
For help with migrating web apps,
see the <a href="/web/dart-2">web app migration guide.</a>
If you’re migrating a Flutter app,
consult the <a href="https://groups.google.com/forum/#!topic/flutter-dev/H8dDhWg_c8I">breaking change notification.</a>
If you publish packages,
then in addition to making platform-specific changes,
follow the <a href="#migrating-packages">package migration instructions below</a>.</p>
<h3 id="general-process">
<a id="general-process" class="anchor" href="#general-process" aria-hidden="true"><span class="octicon octicon-link"></span></a>General process</h3>
<p>Here’s an overview of the process of migrating to Dart 2,
from either Dart 1.x or an earlier version of Dart 2.</p>
<ol>
<li>
<strong>Get an up-to-date version of the Flutter or Dart SDK
and the plugins for any IDEs you use.</strong>
<ul>
<li><a href="https://flutter.dev/docs/development/tools/sdk/upgrading">Flutter SDK upgrade instructions</a></li>
<li>
<a href="/tools/sdk#install">Dart SDK instructions</a> (server-side or web)</li>
</ul>
</li>
<li>
<strong>Upgrade the packages your app depends on.</strong>
<ul>
<li>Flutter: <a href="https://flutter.dev/docs/development/packages-and-plugins/using-packages#updating-package-dependencies"><code class="highlighter-rouge">flutter pub upgrade</code></a>
</li>
<li>Server-side or web: <a href="/guides/packages#upgrading-a-dependency"><code class="highlighter-rouge">pub upgrade</code></a>
</li>
</ul>
</li>
<li>
<strong>Run the <a href="https://github.com/dart-lang/dart2_fix">dart2_fix tool.</a></strong> It helps migrate some
usages of deprecated Dart 1.x APIs to Dart 2.</li>
<li>
<strong>Run the analyzer</strong> to find <a href="/guides/language/sound-problems#static-errors-and-warnings">compile-time errors</a>
and deprecation hints.
<ul>
<li>Flutter: <a href="https://flutter.dev/docs/testing/debugging#the-dart-analyzer"><code class="highlighter-rouge">flutter analyze</code></a>
or use the problems view in Android Studio/IntelliJ or VS Code.</li>
<li>Server-side or web: <a href="/tools/dartanalyzer"><code class="highlighter-rouge">dartanalyzer</code></a>
</li>
</ul>
</li>
<li>
<strong>Fix issues in your code and run the analyzer again</strong>,
repeating until your code passes static analysis.</li>
<li>
<strong>Run tests to find <a href="/guides/language/sound-problems#runtime-errors">runtime errors</a>.</strong>
<ul>
<li>Run all <a href="/guides/testing">automated tests</a> for your software.</li>
<li>Do manual testing, and look for console errors.
Consider adding automated tests to catch issues that you find.</li>
</ul>
</li>
<li><strong>Fix issues until your code works.</strong></li>
<li>
<em>Optional:</em> <strong>Remove <code class="highlighter-rouge">new</code> and unnecessary <code class="highlighter-rouge">const</code>.</strong>
<ul>
<li>You can remove these by hand or use a tool such as <code class="highlighter-rouge">dartfmt --fix</code>.</li>
<li>To find occurrences of <code class="highlighter-rouge">new</code> and unnecessary <code class="highlighter-rouge">const</code>, add the rules
<code class="highlighter-rouge">unnecessary_new</code> and <code class="highlighter-rouge">unnecessary_const</code> to the <code class="highlighter-rouge">linter</code> section of your
<a href="/guides/language/analysis-options#the-analysis-options-file">analysis options file</a>.</li>
</ul>
</li>
</ol>
<h3 id="migrating-packages">
<a id="migrating-packages" class="anchor" href="#migrating-packages" aria-hidden="true"><span class="octicon octicon-link"></span></a>Migrating packages</h3>
<p>As a package owner, you need to do the following:</p>
<ul>
<li>Follow the migration tips for the platforms that your package supports
(see <a href="#migration">above</a>).</li>
<li>Make sure your package passes Dart 2 analysis (see <strong>Run the analyzer</strong> above)</li>
<li>Make sure your package’s users know how to report issues.</li>
<li>Respond quickly to issue reports.</li>
<li>If code changes aren’t backward compatible,
update the lower SDK constraint.</li>
</ul>
<h4 id="changes-and-backward-compatibility">Changes and backward compatibility</h4>
<p>If you have to change your package’s code,
<strong>try to make it work in 1.x</strong>, as well as Dart 2.
For example, you might be able to add type annotations
or (if an API has been removed) to use an alternative 1.x API.</p>
<p>If a backward-compatible change isn’t possible,
<strong>update the lower <a href="/tools/pub/pubspec#sdk-constraints">SDK constraint.</a></strong></p>
<p><a href="/guides/testing">Test your changes</a> to make sure that your package works as expected.</p>
<h4 id="upper-constraint">Upper constraints on the SDK version</h4>
<p>Once your package passes Dart 2 analysis, update the upper constraint
to declare that the package is compatible with Dart 2:</p>
<div class="language-yaml highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="na">environment</span><span class="pi">:</span>
<span class="c1"># Works in Dart 2 only.</span>
<span class="na">sdk</span><span class="pi">:</span> <span class="s1">'</span><span class="s">>=2.0.0</span><span class="nv"> </span><span class="s"><3.0.0'</span>
</code></pre></div></div>
<p>If you plan to maintain compatibility with older versions of Dart, adjust the
lower SDK constraint accordingly:</p>
<div class="language-yaml highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="na">environment</span><span class="pi">:</span>
<span class="c1"># Works in Dart 1 (starting with 1.20.1), and works in Dart 2.</span>
<span class="na">sdk</span><span class="pi">:</span> <span class="s1">'</span><span class="s">>=1.20.1</span><span class="nv"> </span><span class="s"><3.0.0'</span>
</code></pre></div></div>
<p>If you use <a href="https://github.com/dart-lang/sdk/blob/master/CHANGELOG.md">features introduced after 2.0,</a>
be sure to specify the correct lower SDK constraint:</p>
<div class="language-yaml highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="na">environment</span><span class="pi">:</span>
<span class="c1"># Works in 2.1 but not 2.0.</span>
<span class="na">sdk</span><span class="pi">:</span> <span class="s1">'</span><span class="s">>=2.1.0</span><span class="nv"> </span><span class="s"><3.0.0'</span>
</code></pre></div></div>
<aside class="alert alert-warning">
<p><strong>Packages must have an upper constraint of <code class="highlighter-rouge"><3.0.0</code></strong> to work in
Dart 2 stable and subsequent Dart 2 releases.
Dart 2 dev builds <em>before</em> the stable release have
lax upper constraint checking and can use packages that have
no SDK constraints or an upper constraint of <code class="highlighter-rouge"><2.0.0</code>.</p>
</aside>
<h2 id="more-resources">
<a id="more-resources" class="anchor" href="#more-resources" aria-hidden="true"><span class="octicon octicon-link"></span></a>More resources</h2>
<ul>
<li><a href="/tools/dartpad">DartPad</a></li>
<li><a href="/guides/language/spec">Dart Language Specification</a></li>
<li><a href="/tools/sdk#about-release-channels-and-version-strings">About Dart SDK release channels and version strings</a></li>
<li><a href="/tools/pub/pubspec#sdk-constraints">SDK constraints</a></li>
<li>
<a href="https://medium.com/@filiph/updating-your-pub-package-to-dart-2-cd8ca343b1be">Updating your pub package to Dart 2,</a>
an article that includes tips for updating your code and
using Travis to perform continuous integration (CI) testing</li>
</ul>
</div>
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</html>