dart2js cps: Use a global counter to generate hash codes.

pkg/compiler/lib/src/tree_ir/tree_ir_nodes.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 tree_ir_nodes;
 
 import '../constants/values.dart' as values;
 import '../dart_types.dart' show DartType, InterfaceType, TypeVariableType;
 import '../elements/elements.dart';
 import '../io/source_information.dart' show SourceInformation;
@@ skipping 25 matching lines @@
 // In contrast to the CPS-based IR, non-primitive expressions can be named and
 // arguments (to calls, primitives, and blocks) can be arbitrary expressions.
 //
 // Additionally, variables are considered in scope within inner functions;
 // closure variables are thus handled directly instead of using ref cells.
 
 /**
  * The base class of all Tree nodes.
  */
 abstract class Node {
+  /// Workaround for a slow Object.hashCode in the VM.
+  static int _usedHashCodes = 0;
+  final int hashCode = ++_usedHashCodes;
 }
 
 /**
  * The base class of [Expression]s.
  */
 abstract class Expression extends Node {
   accept(ExpressionVisitor v);
   accept1(ExpressionVisitor1 v, arg);
 }
 
@@ skipping 1478 matching lines @@
 
   /// Number of uses of the current fallthrough target.
   int get useCount => _stack.last.useCount;
 
   /// Indicate that a statement will fall through to the current fallthrough
   /// target.
   void use() {
     ++_stack.last.useCount;
   }
 }