Change the default Dart-to-JavaScript compiler in the editor to be dart2js.

client/dart.js

 // Copyright (c) 2012, 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.
 
 // Bootstrap support for Dart scripts on the page as this script.
 if (navigator.webkitStartDart) {
   if (!navigator.webkitStartDart()) {
     document.body.innerHTML = 'This build has expired.  Please download a new Dartium at http://www.dartlang.org/dartium/index.html';
   }
 } else {
   // TODO:
   // - Support in-browser compilation.
   // - Handle inline Dart scripts.
   window.addEventListener("DOMContentLoaded", function (e) {
     // Fall back to compiled JS. Run through all the scripts and
     // replace them if they have a type that indicate that they source
     // in Dart code.
     //
     //   <script type="application/dart" src="<file>.dart"></script>
     //
     // If the script tag has a 'data-compiler' attribute set to
-    // dart2js then we use the dart2js generated file rather than the
-    // one produced by frog:
+    // frog then we use the frog generated file rather than the
+    // one produced by dart2js:
     //
-    //    <script ... data-compiler="dart2js"></script>
+    //    <script ... data-compiler="frog"></script>
     //
     var scripts = document.getElementsByTagName("script");
     var length = scripts.length;
     for (var i = 0; i < length; ++i) {
       if (scripts[i].type == "application/dart") {
         // Remap foo.dart to foo.js or foo.js_ depending
         // on the chosen compiler (frog or dart2js).
         if (scripts[i].src && scripts[i].src != '') {
           var script = document.createElement('script');
           var compiler = scripts[i].getAttribute('data-compiler');
-          if (compiler == "dart2js") {
+          if (compiler == "frog") {
             script.src = scripts[i].src + '.js_';
           } else {
             script.src = scripts[i].src + '.js';
           }
           var parent = scripts[i].parentNode;
           parent.replaceChild(script, scripts[i]);
         }
       }
     }
   }, false);
 }