Remove DartString from constants.

pkg/compiler/lib/src/native/resolver.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.
 
 import 'package:front_end/src/fasta/scanner.dart'
     show BeginGroupToken, StringToken, Token;
 import 'package:front_end/src/fasta/scanner.dart' as Tokens show EOF_TOKEN;
 
 import '../common.dart';
 import '../common_elements.dart' show CommonElements, ElementEnvironment;
@@ skipping 553 matching lines @@
   ConstructedConstantValue constructedObject = value;
   if (constructedObject.type.element != annotationClass) return null;
 
   Iterable<ConstantValue> fields = constructedObject.fields.values;
   // TODO(sra): Better validation of the constant.
   if (fields.length != 1 || fields.single is! StringConstantValue) {
     throw new SpannableAssertionFailure(
         spannable, 'Annotations needs one string: ${value.toStructuredText()}');
   }
   StringConstantValue specStringConstant = fields.single;
-  return specStringConstant.toDartString().slowToString();
+  return specStringConstant.primitiveValue;
 }