Use ConstExp for storing constants.

tests/compiler/dart2js/mirrors_used_test.dart

 // Copyright (c) 2013, 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.
 
 /// Test that the @MirrorsUsed annotation suppress hints and that only
 /// requested elements are retained for reflection.
 library dart2js.test.mirrors_used_test;
 
 import 'package:expect/expect.dart';
 import "package:async_helper/async_helper.dart";
@@ skipping 102 matching lines @@
 
     // There should at least be one metadata constant:
     // 1. The constructed constant for 'MirrorsUsed'.
     Expect.isTrue(backend.metadataConstants.length >= 1);
 
     Set<Constant> compiledConstants = backend.constants.compiledConstants;
     // Make sure that most of the metadata constants aren't included in the
     // generated code.
     for (var dependency in backend.metadataConstants) {
       Constant constant = dependency.constant;
-      Expect.isFalse(compiledConstants.contains(constant), '$constant');
+      Expect.isFalse(compiledConstants.contains(constant),
+                     constant.toStructuredString());
     }
 
     // The type literal 'Foo' is both used as metadata, and as a plain value in
     // the program. Make sure that it isn't duplicated.
     int fooConstantCount = 0;
     for (Constant constant in compiledConstants) {
       if (constant is TypeConstant && '${constant.representedType}' == 'Foo') {
         fooConstantCount++;
       }
     }
@@ skipping 29 matching lines @@
 library lib;
 
 import 'dart:mirrors';
 
 useReflect(type) {
   print(new Symbol('Foo'));
   print(MirrorSystem.getName(reflectClass(type).owner.qualifiedName));
 }
 """,
 };