Add SubclassNode to prepare for optimized queries on ClassWorld.

pkg/compiler/lib/src/dart2jslib.dart

 // Copyright (c) 2012, 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 dart2js;
 
 import 'dart:async';
 import 'dart:collection' show Queue;
 import 'dart:profiler' show
     UserTag;
@@ skipping 41 matching lines @@
 import 'resolution/send_structure.dart';
 import 'resolution/operators.dart' as op;
 import 'scanner/scannerlib.dart';
 import 'serialization/task.dart';
 import 'ssa/ssa.dart';
 import 'io/source_file.dart' show SourceFile;
 import 'tracer.dart' show Tracer;
 import 'tree/tree.dart';
 import 'types/types.dart' as ti;
 import 'universe/universe.dart';
+import 'universe/class_set.dart';
 import 'util/characters.dart' show $_;
 import 'util/uri_extras.dart' as uri_extras show relativize;
 import 'util/util.dart';
 
 export 'helpers/helpers.dart';
 export 'resolution/resolution.dart' show TreeElements, TreeElementMapping;
 export 'scanner/scannerlib.dart' show isUserDefinableOperator,
                                       isUnaryOperator,
                                       isBinaryOperator,
                                       isTernaryOperator,
                                       isMinusOperator;
 export 'universe/universe.dart' show CallStructure, Selector, TypedSelector;
 export 'util/util.dart'
     show Spannable,
          CURRENT_ELEMENT_SPANNABLE,
          NO_LOCATION_SPANNABLE;
 
 part 'compiler.dart';
 part 'diagnostic_listener.dart';
 part 'enqueue.dart';
 part 'resolved_visitor.dart';
 part 'script.dart';
 part 'typechecker.dart';
 part 'warnings.dart';
 part 'world.dart';