Use ConstExp for storing constants.

sdk/lib/_internal/compiler/implementation/dart_backend/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 '../dart2jslib.dart' as dart2js;
 import '../elements/elements.dart';
 import '../universe/universe.dart';
 import '../cps_ir/cps_ir_nodes.dart' as cps_ir;
@@ skipping 173 matching lines @@
   ConcatenateStrings(this.arguments, [this.constant]);
 
   accept(ExpressionVisitor visitor) => visitor.visitConcatenateStrings(this);
 }
 
 /**
  * A constant.
  */
 class Constant extends Expression {
   final ConstExp expression;
-  final dart2js.Constant value;
 
-  Constant(this.expression, this.value);
+  Constant(this.expression);
 
   Constant.primitive(dart2js.PrimitiveConstant primitiveValue)
-      : expression = new PrimitiveConstExp(primitiveValue),
-        value = primitiveValue;
+      : expression = new PrimitiveConstExp(primitiveValue);
 
   accept(ExpressionVisitor visitor) => visitor.visitConstant(this);
+
+  dart2js.Constant get value => expression.value;
 }
 
 class This extends Expression {
   accept(ExpressionVisitor visitor) => visitor.visitThis(this);
 }
 
 class ReifyTypeVar extends Expression {
   TypeVariableElement typeVariable;
 
   ReifyTypeVar(this.typeVariable);
@@ skipping 433 matching lines @@
   visitFunctionDeclaration(FunctionDeclaration node) {
     visitFunctionDefinition(node.definition);
     visitStatement(node.next);
   }
 
   visitExpressionStatement(ExpressionStatement node) {
     visitExpression(node.expression);
     visitStatement(node.next);
   }
 }