web_components: fix extending another custom element

pkg/web_components/lib/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;
@@ skipping 11 matching lines @@
 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/dart_support.js` is loaded and available "
         "before calling this function.");
   }
 
   var upgrader = document.createElementUpgrader(
       dartType, extendsTag: extendsTag);
-  _doc.callMethod('_registerDartTypeUpgrader', [tagName, upgrader.upgrade]);
+
+  // 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');