Change how we check for user agent, in preparation for breaking change in Dartium.

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) {
+  // TODO(sigmund): remove the userAgent check once 1.6 rolls as stable.
+  // See: dartbug.com/18463
+  if (navigator.dartEnabled || (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.");
   }
 
   var needsConstructorFix = window.constructor === window.Window;
 
   // TODO(jmesserly): we need to wrap document somehow (a dart:html hook?)
 
   // dartNativeDispatchHooksTransformer is described on initHooks() in
@@ skipping 156 matching lines @@
           return originalGetUnknownTag(o, tag);
         }
       };
     });
   }
 
   doc._registerDartTypeUpgrader = registerDartTypeUpgrader;
   doc._onlyUpgradeNewElements = onlyUpgradeNewElements;
   doc.registerElement = registerElement;
 })(document);