title: update dart sdk for flutter
body: <p>I would like to use dart SDK >= 2.2.0 with flutter. But my current version used BY Flutter is 2.1.2</p>
  
  <pre><code>flutter --version
  Flutter 1.2.1 • channel stable • https://github.com/flutter/flutter.git
  Framework • revision 8661d8aecd (2 months ago) • 2019-02-14 19:19:53 -0800
  Engine • revision 3757390fa4
  Tools • Dart 2.1.2 (build 2.1.2-dev.0.0 0a7dcf17eb)
  </code></pre>
  
  <p>I tried to install the 2.2.0 version independently and I succeed :</p>
  
  <pre><code>dart --version
  Dart VM version: 2.2.0 (Tue Feb 26 15:04:32 2019 +0100) on "macos_x64"
  </code></pre>
  
  <p>However, Flutter doesn't use this version as you can see above. I tried to replace files of the <code>dart-sdk</code> used by flutter (<code>flutter/bin/cache/dart-sdk</code>) by the version that I installed independently, but when I try to run Flutter after that I have a snapshot problem so I have put back the original <code>dart-sdk</code> folder in the flutter directory.</p>
  
  <p>Do you have any ideas how can I update it? </p>
  
  <p>PS: I downloaded flutter very recently (10 days ago) from here: <a href="https://flutter.dev/docs/get-started/install/macos" rel="noreferrer">https://flutter.dev/docs/get-started/install/macos</a></p>