dart2js: get:hashCode methods to avoid Object.hashCode

pkg/compiler/lib/src/resolution/tree_elements.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.
 
 library dart2js.resolution.tree_elements;
 
 import '../common.dart';
 import '../constants/expressions.dart';
 import '../dart_types.dart';
 import '../diagnostics/source_span.dart';
@@ skipping 121 matching lines @@
 
   /// Map from labels to their label definition.
   Map<Label, LabelDefinition> _definedLabels;
 
   /// Map from labeled goto statements to the labels they target.
   Map<GotoStatement, LabelDefinition> _targetLabels;
 
   /// Map from nodes to native data.
   Map<Node, dynamic> _nativeData;
 
-  final int hashCode = ++_hashCodeCounter;
+  final int hashCode = _hashCodeCounter = (_hashCodeCounter + 1).toUnsigned(30);
   static int _hashCodeCounter = 0;
 
   TreeElementMapping(this.analyzedElement);
 
   operator []=(Node node, Element element) {
     // TODO(johnniwinther): Simplify this invariant to use only declarations in
     // [TreeElements].
     assert(invariant(node, () {
       if (!element.isMalformed && analyzedElement != null && element.isPatch) {
         return analyzedElement.implementationLibrary.isPatch;
@@ skipping 368 matching lines @@
       _nativeData = <Node, dynamic>{};
     }
     _nativeData[node] = nativeData;
   }
 
   @override
   dynamic getNativeData(Node node) {
     return _nativeData != null ? _nativeData[node] : null;
   }
 }