Add observatory_pub_packages snapshot to third_party

observatory_pub_packages/web_components/interop.dart

+// 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.
+
+/// Provides support for associating a Dart type for Javascript Custom Elements.
+/// This will not work unless `dart_support.js` is loaded.
+library web_components.interop;
+
+import 'dart:async' show Stream, StreamController;
+import 'dart:html' show document, Element;
+import 'dart:js' show JsObject, JsFunction;
+
+final _doc = new JsObject.fromBrowserObject(document);
+
+/// Returns whether [registerDartType] is supported, which requires to have
+/// `dart_support.js` already loaded in the page.
+bool get isSupported => _doc.hasProperty('_registerDartTypeUpgrader');
+
+/// Watches when Javascript custom elements named [tagName] are created and
+/// associates the created element with the given [dartType]. Only one Dart type
+/// can be registered for a given tag name.
+void registerDartType(String tagName, Type dartType, {String extendsTag}) {
+  if (!isSupported) {
+    throw new UnsupportedError("Couldn't find "
+        "`document._registerDartTypeUpgrader`. Please make sure that "
+        "`packages/web_components/interop_support.html` is loaded and "
+        "available before calling this function.");
+  }
+
+  var upgrader = document.createElementUpgrader(
+      dartType, extendsTag: extendsTag);
+
+  // Unfortunately the dart:html upgrader will throw on an already-upgraded
+  // element, so we need to duplicate the type check to prevent that.
+  // An element can be upgraded twice if it extends another element and calls
+  // createdCallback on the superclass. Since that's a valid use case we must
+  // wrap at both levels, and guard against it here.
+  upgradeElement(e) {
+    if (e.runtimeType != dartType) upgrader.upgrade(e);
+  }
+
+  _doc.callMethod('_registerDartTypeUpgrader', [tagName, upgradeElement]);
+}
+
+/// This function is mainly used to save resources. By default, we save a log of
+/// elements that are created but have no Dart type associated with them. This
+/// is so we can upgrade them as soon as [registerDartType] is invoked. This
+/// function can be called to indicate that we no longer are interested in
+/// logging element creations and that it is sufficient to only upgrade new
+/// elements as they are being created. Typically this is called after the last
+/// call to [registerDartType] or as soon as you know that no element will be
+/// created until the call to [registerDartType] is made.
+void onlyUpgradeNewElements() => _doc.callMethod('_onlyUpgradeNewElements');