Add "created-watcher" to the web_components package. This was adapted from

pkg/web_components/lib/dart_support.js

 // Copyright (c) 2014, 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.
 
+// Teaches dart2js about the wrapping that is done by the Shadow DOM polyfill.
 (function() {
   var ShadowDOMPolyfill = window.ShadowDOMPolyfill;
   if (!ShadowDOMPolyfill) return;
 
   if (navigator.userAgent.indexOf('(Dart)') !== -1) {
     console.error("ShadowDOMPolyfill polyfill was loaded in Dartium. This " +
         "will not work. This indicates that Dartium's Chrome version is " +
         "not compatible with this version of web_components.");
   }
 
@@ skipping 41 matching lines @@
           }
           return name;
         }
 
         obj = unwrapped;
       }
       return originalGetTag(obj);
     }
   });
 })();