Add observatory_pub_packages snapshot to third_party

observatory_pub_packages/web_components/polyfill.dart

Index: observatory_pub_packages/web_components/polyfill.dart
===================================================================
--- observatory_pub_packages/web_components/polyfill.dart	(revision 0)
+++ observatory_pub_packages/web_components/polyfill.dart	(working copy)
@@ -0,0 +1,61 @@
+// Copyright (c) 2013, 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.
+
+/** Dart APIs for interacting with the JavaScript Custom Elements polyfill. */
+library web_components.polyfill;
+
+import 'dart:async';
+import 'dart:html';
+import 'dart:js' as js;
+
+/**
+ * A future that completes once all custom elements in the initial HTML page
+ * have been upgraded.
+ *
+ * This is needed because the native implementation can update the elements
+ * 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();
+
+  // Not upgraded. Wait for the polyfill to fire the WebComponentsReady event.
+  // Note: we listen on document (not on document.body) to allow this polyfill
+  // to be loaded in the HEAD element.
+  return document.on['WebComponentsReady'].first;
+}();
+
+// Return true if we are using the polyfill and upgrade is complete, or if we
+// have native document.register and therefore the browser took care of it.
+// Otherwise return false, including the case where we can't find the polyfill.
+bool get _isReady {
+  // If we don't have dart:js, assume things are ready
+  if (js.context == null) return true;
+
+  var customElements = js.context['CustomElements'];
+  if (customElements == null) {
+    // Return true if native document.register, otherwise false.
+    // (Maybe the polyfill isn't loaded yet. Wait for it.)
+    return document.supportsRegister;
+  }
+
+  return customElements['ready'] == true;
+}
+
+/**
+ * *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.
+ *
+ * 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.
+ */
+void customElementsTakeRecords([Node node]) {
+  var customElements = js.context['CustomElements'];
+  if (customElements == null) return;
+  customElements.callMethod('takeRecords', [node]);
+}