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

pkg/compiler/lib/src/js_emitter/program_builder/program_builder.dart

Index: pkg/compiler/lib/src/js_emitter/program_builder/program_builder.dart
diff --git a/pkg/compiler/lib/src/js_emitter/program_builder/program_builder.dart b/pkg/compiler/lib/src/js_emitter/program_builder/program_builder.dart
index 3b15ff69d3b76372190a75bcbdf1c78b52be32c8..b4a9c37988f063303fc6cdf245149ddeeb8bff66 100644
--- a/pkg/compiler/lib/src/js_emitter/program_builder/program_builder.dart
+++ b/pkg/compiler/lib/src/js_emitter/program_builder/program_builder.dart
@@ -325,7 +325,7 @@ class ProgramBuilder {
   }
 
   Class _buildClass(ClassElement element) {
-    bool onlyForRti = _task.typeTestRegistry.rtiNeededClasses.contains(element);
+    bool onlyForRti = _task.classesOnlyNeededForRti.contains(element);
 
     List<Method> methods = [];
     List<StubMethod> callStubs = <StubMethod>[];