Type tracing bug fix for issue 28919.

tests/compiler/dart2js/mock_libraries.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.
 
 // Library for creating mock versions of platform and internal libraries.
 
 library mock_libraries;
 
 const DEFAULT_PLATFORM_CONFIG = """
 [libraries]
@@ skipping 286 matching lines @@
         get hashCode => throw "Interceptor.hashCode not implemented.";
         noSuchMethod(im) { throw im; }
       }''',
   'JSArray': r'''
           class JSArray<E> extends Interceptor implements List<E>, JSIndexable {
             JSArray();
             factory JSArray.typed(a) => a;
             var length;
             operator[](index) => this[index];
             operator[]=(index, value) { this[index] = value; }
-            add(value) { this[length + 1] = value; }
+            add(value) { this[length] = value; }
             insert(index, value) {}
             E get first => this[0];
             E get last => this[0];
             E get single => this[0];
             E removeLast() => this[0];
             E removeAt(index) => this[0];
             E elementAt(index) => this[0];
             E singleWhere(f) => this[0];
-            Iterator<E> get iterator => null; 
+            Iterator<E> get iterator => null;
           }''',
   'JSBool': 'class JSBool extends Interceptor implements bool {}',
   'JSDouble': 'class JSDouble extends JSNumber implements double {}',
   'JSExtendableArray': 'class JSExtendableArray extends JSMutableArray {}',
   'JSFixedArray': 'class JSFixedArray extends JSMutableArray {}',
   'JSFunction':
       'abstract class JSFunction extends Interceptor implements Function {}',
   'JSIndexable': r'''
       abstract class JSIndexable {
         get length;
@@ skipping 135 matching lines @@
 
     const LookupMap(this._entries, [this._nestedMaps = const []])
         : _key = null, _value = null;
 
     const LookupMap.pair(this._key, this._value)
         : _entries = const [], _nestedMaps = const [];
     V operator[](K k) => null;
   }''',
   '_version': 'const _version = "0.0.1+1";',
 };