Decouple TypeMask from ClassElement

pkg/compiler/lib/src/types/masks.dart

 // Copyright (c) 2016, 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 masks;
 
 import '../common.dart';
 import '../common/backend_api.dart' show BackendClasses;
 import '../compiler.dart' show Compiler;
 import '../constants/values.dart' show PrimitiveConstantValue;
 import '../elements/elements.dart';
 import '../inferrer/type_graph_inferrer.dart' show TypeGraphInferrer;
 import '../tree/tree.dart';
 import '../universe/selector.dart' show Selector;
 import '../universe/world_builder.dart'
     show
         ReceiverConstraint,
         UniverseSelectorConstraints,
         SelectorConstraintsStrategy;
 import '../util/util.dart';
-import '../world.dart' show ClosedWorld;
+import '../world.dart' show ClassQuery, ClosedWorld;
 import 'abstract_value_domain.dart' show AbstractValue;
 
 part 'container_type_mask.dart';
 part 'dictionary_type_mask.dart';
 part 'flat_type_mask.dart';
 part 'forwarding_type_mask.dart';
 part 'map_type_mask.dart';
 part 'type_mask.dart';
 part 'union_type_mask.dart';
 part 'value_type_mask.dart';
@@ skipping 96 matching lines @@
       _asyncFutureType ??= new TypeMask.nonNullExact(
           backendClasses.asyncFutureImplementation, closedWorld);
 
   TypeMask get asyncStarStreamType =>
       _asyncStarStreamType ??= new TypeMask.nonNullExact(
           backendClasses.asyncStarStreamImplementation, closedWorld);
 
   // TODO(johnniwinther): Assert that the null type has been resolved.
   TypeMask get nullType => _nullType ??= const TypeMask.empty();
 }