Add suppression to test.dart for flaky content_shell crashes

tools/testing/dart/browser_controller.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 library browser;
 
 import "dart:async";
-import "dart:convert" show LineSplitter, UTF8, JSON;
+import "dart:convert" show UTF8, JSON;
 import "dart:core";
 import "dart:io";
-import "dart:math" show max, min;
+import "dart:math" show min;
 
 import 'android.dart';
 import 'http_server.dart';
 import 'path.dart';
 import 'utils.dart';
 
 import 'reset_safari.dart' show
     killAndResetSafari;
 
 class BrowserOutput {
@@ skipping 1828 matching lines @@
     </div>
     <div id="embedded_iframe_div" class="test box">
       <iframe id="embedded_iframe"></iframe>
     </div>
   </body>
 </html>
 """;
     return driverContent;
   }
 }