Update to use one less proxy per element

test/src/common/behavior_composition_test.dart

 // Copyright (c) 2015, 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.
 @TestOn('browser')
 library polymer.test.src.common.behavior_composition_test;
 
 import 'dart:html';
 import 'dart:js';
 import 'package:polymer/polymer.dart';
 import 'package:polymer/src/common/polymer_descriptor.dart';
 import 'package:test/test.dart';
 import 'package:web_components/web_components.dart';
 import '../../common.dart';
 
 main() async {
   await initPolymer();
 
   group('behavior composition', () {
     test('Correct usage of mixins works', () {
       var descriptor =
           createPolymerDescriptor(Good, const PolymerRegister('good-element'));
-      // Should only get [BehaviorFour] in the top level behaviors object.
-      expect(descriptor['behaviors'].length, 1);
+      // Should only get [BehaviorFour] in the top level behaviors object, in
+      // addition to Polymer.Dart.Behavior.
+      expect(descriptor['behaviors'].length, 2);
 
       var behaviorFour = behavior.getBehavior(BehaviorFour);
-      expect(descriptor['behaviors'][0], behaviorFour);
+      expect(descriptor['behaviors'][1], behaviorFour);
       expect(behaviorFour, new isInstanceOf<JsArray>());
       expect(behaviorFour.length, 2);
 
       var behaviorThree = behavior.getBehavior(BehaviorThree);
       expect(behaviorFour[0], behaviorThree);
       expect(behaviorFour[1], new isInstanceOf<JsObject>());
       expect(behaviorThree, new isInstanceOf<JsArray>());
       expect(behaviorThree.length, 3);
 
       var behaviorTwo = behavior.getBehavior(BehaviorTwo);
@@ skipping 40 matching lines @@
 }
 
 class Incomplete extends PolymerElement with BehaviorOne, BehaviorThree {
   Incomplete.created() : super.created();
 }
 
 class Invalid extends PolymerElement
     with BehaviorTwo, BehaviorOne, BehaviorThree {
   Invalid.created() : super.created();
 }