Remove DartString from constants.

pkg/compiler/lib/src/deferred_load.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 deferred_load;
 
 import 'common/tasks.dart' show CompilerTask;
 import 'common.dart';
 import 'compiler.dart' show Compiler;
 import 'constants/expressions.dart' show ConstantExpression;
@@ skipping 1062 matching lines @@
       assert(metadatas != null);
       for (MetadataAnnotation metadata in metadatas) {
         metadata.ensureResolved(compiler.resolution);
         ConstantValue value =
             compiler.constants.getConstantValue(metadata.constant);
         ResolutionDartType type = value.getType(compiler.commonElements);
         Element element = type.element;
         if (element == compiler.commonElements.deferredLibraryClass) {
           ConstructedConstantValue constant = value;
           StringConstantValue s = constant.fields.values.single;
-          result = s.primitiveValue.slowToString();
+          result = s.primitiveValue;
           break;
         }
       }
     }
     assert(result != null);
     return result;
   }
 
   bool operator ==(other) {
     if (other is! _DeclaredDeferredImport) return false;
     return declaration == other.declaration;
   }
 
   int get hashCode => declaration.hashCode * 17;
 
   String toString() => '$declaration';
 }