Add SubclassNode to prepare for optimized queries on ClassWorld.

pkg/compiler/lib/src/use_unused_api.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.
 
 /// This file use methods that aren't used by dart2js.dart, but that we wish to
 /// keep anyway. This might be general API that isn't currently in use,
 /// debugging aids, or API only used for testing (see TODO below).
 
 library dart2js.use_unused_api;
 
@@ skipping 242 matching lines @@
 usedByTests() {
   // TODO(ahe): We should try to avoid including API used only for tests. In
   // most cases, such API can be moved to a test library.
   dart2jslib.World world = null;
   dart2jslib.Compiler compiler = null;
   compiler.currentlyInUserCode();
   type_graph_inferrer.TypeGraphInferrer typeGraphInferrer = null;
   source_file_provider.SourceFileProvider sourceFileProvider = null;
   sourceFileProvider.getSourceFile(null);
   world.hasAnyUserDefinedGetter(null, null);
+  world.subclassesOf(null);
+  world.classHierarchyNode(null);
   typeGraphInferrer.getCallersOf(null);
   dart_types.Types.sorted(null);
   new dart_types.Types(compiler).copy(compiler);
   sourceFileProvider.readStringFromUri(null);
 }
 
 useElements(
     [elements.ClassElement e,
      elements.Name n,
      modelx.FieldElementX f,
@@ skipping 55 matching lines @@
 
 class TreeVisitor1 extends tree_ir.ExpressionVisitor1
                       with tree_ir.StatementVisitor1 {
   noSuchMethod(inv) {}
 }
 
 useTreeVisitors() {
   new TreeVisitor1().visitExpression(null, null);
   new TreeVisitor1().visitStatement(null, null);
 }