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

pkg/polymer/lib/boot.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.
 
 /// Experimental bootstrap to initialize polymer applications. This library is
 /// not used by default, and may be replaced by Dart code in the near future.
 ///
 /// This script contains logic to bootstrap polymer apps during development. It
 /// internally discovers Dart script tags through HTML imports, and constructs
 /// a new entrypoint for the application that is then launched in an isolate.
@@ skipping 15 matching lines @@
 /// script tags in the main page, so you may need to move some code into an HTML
 /// import. For example, If you need to run some initialization code before any
 /// other code is executed, include an HTML import to an html file with a
 /// "application/dart" script tag that contains an initializer
 /// method with the body of your old main, and make sure this tag is placed
 /// above other html-imports that load the rest of the application.
 /// Initialization methods are executed in the order in which they are
 /// discovered in the HTML document.
 (function() {
   // Only run in Dartium.
-  if (navigator.userAgent.indexOf('(Dart)') === -1) return;
+  if (!navigator.dartEnabled &&
+      // TODO(sigmund): remove userAgent check once 1.6 rolls as stable.
+      // See: dartbug.com/18463
+      (navigator.userAgent.indexOf('(Dart)') === -1)) {
+    return;
+  }
 
   // Extract a Dart import URL from a script tag, which is the 'src' attribute
   // of the script tag, or a data-url with the script contents for inlined code.
   function getScriptUrl(script) {
     var url = script.src;
     if (url) {
       // Normalize package: urls
       var index = url.indexOf('packages/');
       if (index == 0 || (index > 0 && url[index - 1] == '/')) {
         url = "package:" + url.slice(index + 9);
@@ skipping 72 matching lines @@
         + 'doesn\'t have a main, but a top-level method marked with '
         + '@initMethod instead');
       for (var i = 0; i < results.badTags.length; i++) {
         console.warn(results.badTags[i]);
       }
     }
     newScript.textContent = createMain(results.scripts);
     document.body.appendChild(newScript);
   });
 })();