Use ConstExp for storing constants.

sdk/lib/_internal/compiler/implementation/dart_backend/tree_ir_nodes.dart

Index: sdk/lib/_internal/compiler/implementation/dart_backend/tree_ir_nodes.dart
diff --git a/sdk/lib/_internal/compiler/implementation/dart_backend/tree_ir_nodes.dart b/sdk/lib/_internal/compiler/implementation/dart_backend/tree_ir_nodes.dart
index fd3f677e7e27d25e4ea59157a7fc2517d4141e64..f9f43777c98af1a6f65541d42030e8b532ebdd4d 100644
--- a/sdk/lib/_internal/compiler/implementation/dart_backend/tree_ir_nodes.dart
+++ b/sdk/lib/_internal/compiler/implementation/dart_backend/tree_ir_nodes.dart
@@ -191,15 +191,15 @@ class ConcatenateStrings extends Expression {
  */
 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 {