dart2js: Make it more obvious how rti-only needed classes work.

pkg/compiler/lib/src/js_emitter/program_builder/program_builder.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 dart2js.js_emitter.program_builder;
 
 import '../js_emitter.dart' show computeMixinClass;
 import '../model.dart';
 
 import '../../common.dart';
@@ skipping 307 matching lines @@
     js.Name name = namer.className(element);
 
     return new Class(
         element, name, null, [], instanceFields, [], [], [], [], [], null,
         isDirectlyInstantiated: true,
         onlyForRti: false,
         isNative: element.isNative);
   }
 
   Class _buildClass(ClassElement element) {
-    bool onlyForRti = _task.typeTestRegistry.rtiNeededClasses.contains(element);
+    bool onlyForRti = _task.classesOnlyNeededForRti.contains(element);
 
     List<Method> methods = [];
     List<StubMethod> callStubs = <StubMethod>[];
 
     ClassStubGenerator classStubGenerator =
         new ClassStubGenerator(_compiler, namer, backend);
     RuntimeTypeGenerator runtimeTypeGenerator =
         new RuntimeTypeGenerator(_compiler, _task, namer);
 
     void visitMember(ClassElement enclosing, Element member) {
@@ skipping 423 matching lines @@
       _registry.registerConstant(outputUnit, constantValue);
       assert(!_constants.containsKey(constantValue));
       js.Name name = namer.constantName(constantValue);
       String constantObject = namer.globalObjectForConstant(constantValue);
       Holder holder = _registry.registerHolder(constantObject);
       Constant constant = new Constant(name, holder, constantValue);
       _constants[constantValue] = constant;
     }
   }
 }