Use ConstExp for storing constants.

sdk/lib/_internal/compiler/implementation/dart_backend/tree_ir_builder.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_builder;
 
 import '../dart2jslib.dart' as dart2js;
 import '../elements/elements.dart';
 import '../cps_ir/cps_ir_nodes.dart' as cps_ir;
 import 'tree_ir_nodes.dart';
@@ skipping 391 matching lines @@
     thenStatement =
         cont.hasExactlyOneUse ? visit(cont.body) : new Break(labels[cont]);
     cont = node.falseContinuation.definition;
     assert(cont.parameters.isEmpty);
     elseStatement =
         cont.hasExactlyOneUse ? visit(cont.body) : new Break(labels[cont]);
     return new If(condition, thenStatement, elseStatement);
   }
 
   Expression visitConstant(cps_ir.Constant node) {
-    return new Constant(node.expression, node.value);
+    return new Constant(node.expression);
   }
 
   Expression visitThis(cps_ir.This node) {
     return new This();
   }
 
   Expression visitReifyTypeVar(cps_ir.ReifyTypeVar node) {
     return new ReifyTypeVar(node.typeVariable);
   }
 
@@ skipping 39 matching lines @@
     // visited.
     compiler.internalError(compiler.currentElement, 'Unexpected IR node.');
     return null;
   }
 
   Expression visitIsTrue(cps_ir.IsTrue node) {
     return getVariableReference(node.value);
   }
 }