Allow developers to try out dart2js in their web apps.

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) {
   navigator.webkitStartDart();
 } else {
+  // TODO:
+  // - Support in-browser compilation.
+  // - Handle inline Dart scripts.
   window.addEventListener("DOMContentLoaded", function (e) {
-      // Fall back to compiled JS.
-      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.
-          // TODO:
-          // - Support in-browser compilation.
-          // - Handle inline Dart scripts.
-          if (scripts[i].src && scripts[i].src != '') {
-            var script = document.createElement('script');
+    // 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:
+    //
+    //    <script ... data-compiler="dart2js"></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") {
+            script.src = scripts[i].src + '.js_';
+          } else {
             script.src = scripts[i].src + '.js';
-            var parent = scripts[i].parentNode;
-            parent.replaceChild(script, scripts[i]);
           }
+          var parent = scripts[i].parentNode;
+          parent.replaceChild(script, scripts[i]);
         }
       }
-    }, false);
+    }
+  }, false);
 }