introduce web_components pkg for consolidated polyfills

pkg/web_components/lib/polyfill.dart

Index: pkg/web_components/lib/polyfill.dart
diff --git a/pkg/custom_element/lib/polyfill.dart b/pkg/web_components/lib/polyfill.dart
similarity index 69%
rename from pkg/custom_element/lib/polyfill.dart
rename to pkg/web_components/lib/polyfill.dart
index 413d7d699a04f0e2acb736234d887a811520642e..a141c4d60860ca170e31a6e0c140e83bca5a85de 100644
--- a/pkg/custom_element/lib/polyfill.dart
+++ b/pkg/web_components/lib/polyfill.dart
@@ -3,7 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 
 /** Dart APIs for interacting with the JavaScript Custom Elements polyfill. */
-library custom_element.polyfill;
+library web_components.polyfill;
 
 import 'dart:async';
 import 'dart:html';
@@ -17,6 +17,7 @@ import 'dart:js' as js;
  * while parsing the HTML document, but the custom element polyfill cannot,
  * so it completes this future once all elements are upgraded.
  */
+// TODO(jmesserly): rename to webComponentsReady to match the event?
 Future customElementsReady = () {
   if (_isReady) return new Future.value();
 
@@ -44,20 +45,18 @@ bool get _isReady {
 }
 
 /**
- * Loads `custom-elements.debug.js` or `custom-elements.min.js` by adding the
- * script tag to the page. Returns a future that completes when custom elements
- * are ready (equivalent to [customElementsReady]).
+ * *Note* this API is primarily intended for tests. In other code it is better
+ * to write it in a style that works with or without the polyfill, rather than
+ * using this method.
  *
- * Normally you should add this to your HTML file
- * (the Polymer package will do this automatically), but loading dynamically
- * can be useful for scenarios such as tests.
+ * Synchronously trigger evaluation of pending lifecycle events, which otherwise
+ * need to wait for a [MutationObserver] to signal the changes in the polyfill.
+ * This method can be used to resolve differences in timing between native and
+ * polyfilled custom elements.
  */
-Future loadCustomElementPolyfill() {
-  if (!document.supportsRegister && !js.context.hasProperty('CustomElements')) {
-    var script = new ScriptElement()
-        ..src = '/packages/custom_element/custom-elements.debug.js';
-    document.head.append(script);
-    return document.on['WebComponentsReady'].first;
+void customElementsTakeRecords() {
+  var customElements = js.context['CustomElements'];
+  if (customElements != null) {
+    customElements.callMethod('takeRecords');
   }
-  return new Future.value();
 }