Turn on Polymer TodoMVC tests on Dart buildbots

pkg/unittest/lib/test_controller.js

 // Copyright (c) 2011, 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.
 
 /**
  * Test controller logic - used by unit test harness to embed tests in
  * conent shell.
  */
 
 // Clear the console before every test run - this is Firebug specific code.
 if (typeof console == "object" && typeof console.clear == "function") {
   console.clear();
 }
 // Set window onerror to make sure that we catch test harness errors across all
 // browsers.
 window.onerror = function (message, url, lineNumber) {
   if (url) {
     showErrorAndExit(
         "\n\n" + url + ":" + lineNumber + ":\n" + message + "\n\n");
   } else {
     showErrorAndExit(message);
   }
   window.postMessage('unittest-suite-external-error', '*');
 };
 
-if (navigator.webkitStartDart) {
+// Start Dartium/content_shell, unless we are waiting for HTML Imports to load.
+// HTML Imports allows a document to link to other HTMLs documents via
+// <link rel=import>. It also allows for those other documents to contain
+// <script> tags, which must be run before scripts on the main page.
+// We have package:html_import to polyfill this feature, and it will handle
+// starting Dartium/content_shell in that case. HTML Imports is used by Polymer,
+// but it could be used by itself too. See the specification:
+// https://dvcs.w3.org/hg/webcomponents/raw-file/tip/spec/imports/index.html
+if (navigator.webkitStartDart && !window.HTMLImports) {
   navigator.webkitStartDart();
 }
 
 // testRunner is provided by content shell.
 // It is not available in selenium tests.
 var testRunner = window.testRunner || window.layoutTestController;
 
 var waitForDone = false;
 
 // Returns the driving window object if available
@@ skipping 127 matching lines @@
   try {
     main();
   } catch (e) {
     dartPrint(e);
     if (e.stack) dartPrint(e.stack);
     window.postMessage('unittest-suite-fail', '*');
     return;
   }
   window.postMessage('dart-main-done', '*');
 }