Avoid Object.hashCode on a few objects

pkg/compiler/lib/src/closure.dart

 // Copyright (c) 2012, 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 closureToClassMapper;
 
 import 'common.dart';
 import 'common/names.dart' show
     Identifiers;
 import 'common/resolution.dart' show
@@ skipping 291 matching lines @@
     throw new UnsupportedError("BoxFieldElement.resolvedAst");
   }
 
   @override
   ConstantExpression get constant => null;
 }
 
 /// A local variable used encode the direct (uncaptured) references to [this].
 class ThisLocal extends Local {
   final ExecutableElement executableContext;
+  final hashCode = ++ElementX.elementHashCode;
 
   ThisLocal(this.executableContext);
 
   String get name => 'this';
 
   ClassElement get enclosingClass => executableContext.enclosingClass;
 }
 
 /// Call method of a closure class.
 class SynthesizedCallMethodElementX extends BaseFunctionElementX
@@ skipping 835 matching lines @@
 ///
 /// Move the below classes to a JS model eventually.
 ///
 abstract class JSEntity implements Entity {
   Entity get declaredEntity;
 }
 
 abstract class PrivatelyNamedJSEntity implements JSEntity {
   Entity get rootOfScope;
 }