Revert "Revert "Dart2js support for custom element lifecycle events""

tests/html/utils.dart

Index: tests/html/utils.dart
diff --git a/tests/html/utils.dart b/tests/html/utils.dart
index 2533f24997b2a34e3694967f82611187efaafa29..04c8235353f0303d9a0ce03b5971b9d6f0414c15 100644
--- a/tests/html/utils.dart
+++ b/tests/html/utils.dart
@@ -159,3 +159,17 @@ void validateNodeTree(Node a, Node b, [String path = '']) {
     validateNodeTree(a.nodes[i], b.nodes[i], '$path[$i].');
   }
 }
+
+Future loadCustomElementPolyfill() {
+  if (!document.supportsRegister) {
+    var script = new ScriptElement()
+        ..src = '/packages/custom_element/custom-elements.debug.js';
+    document.head.append(script);
+    return document.body.on['WebComponentsReady'].first;
+  }
+  return new Future.value();
+}
+
+Future loadPolyfills() {
+  return loadCustomElementPolyfill();
+}