Remove DartString from constants.

tests/compiler/dart2js/deferred_follow_constant_dependencies_test.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.
 
 // Test that constants depended on by other constants are correctly deferred.
 
 import 'package:async_helper/async_helper.dart';
 import 'package:compiler/src/common.dart';
 import 'package:compiler/src/constants/values.dart';
 import 'package:compiler/src/compiler.dart';
@@ skipping 14 matching lines @@
     List<ConstantValue> allConstants = [];
 
     addConstantWithDependendencies(ConstantValue c) {
       allConstants.add(c);
       c.getDependencies().forEach(addConstantWithDependendencies);
     }
 
     backend.constants.compiledConstants.forEach(addConstantWithDependendencies);
     for (String stringValue in ["cA", "cB", "cC"]) {
       ConstantValue constant = allConstants.firstWhere((constant) {
-        return constant.isString &&
-            constant.primitiveValue.slowToString() == stringValue;
+        return constant.isString && constant.primitiveValue == stringValue;
       });
       Expect.notEquals(null, outputUnitForConstant(constant),
           "Constant value ${constant.toStructuredText()} has no output unit.");
       Expect.notEquals(
           mainOutputUnit,
           outputUnitForConstant(constant),
           "Constant value ${constant.toStructuredText()} "
           "is in the main output unit.");
     }
   });
@@ skipping 19 matching lines @@
 """,
   "lib.dart": """
 class C {
   final a;
   const C(this.a);
 }
 
 const L = const {"cA": const C(const {"cB": "cC"})};
 """,
 };