Move towards using WorldImpact for codegen

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 Backend;
 import '../compiler.dart' show Compiler;
 import '../constants/values.dart' show PrimitiveConstantValue;
@@ skipping 19 matching lines @@
 part 'union_type_mask.dart';
 part 'value_type_mask.dart';
 
 class CommonMasks {
   final ClassWorld classWorld;
   // TODO(sigmund): once we split out the backend common elements, depend
   // directly on those instead.
   final Compiler compiler;
 
   CommonMasks(Compiler compiler)
-      : this.classWorld = compiler.closedWorld,
+      : this.classWorld = compiler.openWorld,
         compiler = compiler;
 
   TypeMask _dynamicType;
   TypeMask _nonNullType;
   TypeMask _nullType;
   TypeMask _intType;
   TypeMask _uint32Type;
   TypeMask _uint31Type;
   TypeMask _positiveIntType;
   TypeMask _doubleType;
@@ skipping 76 matching lines @@
       _asyncFutureType ??= new TypeMask.nonNullExact(
           compiler.backend.asyncFutureImplementation, classWorld);
 
   TypeMask get asyncStarStreamType =>
       _asyncStarStreamType ??= new TypeMask.nonNullExact(
           compiler.backend.asyncStarStreamImplementation, classWorld);
 
   // TODO(johnniwinther): Assert that the null type has been resolved.
   TypeMask get nullType => _nullType ??= const TypeMask.empty();
 }