dart2js: add comments.

pkg/compiler/lib/src/js_emitter/old_emitter/type_test_emitter.dart

Index: pkg/compiler/lib/src/js_emitter/old_emitter/type_test_emitter.dart
diff --git a/pkg/compiler/lib/src/js_emitter/old_emitter/type_test_emitter.dart b/pkg/compiler/lib/src/js_emitter/old_emitter/type_test_emitter.dart
index aa88a7a043599636f34fe8835510a72a04921424..6b4ff3d7762ca899b3d571070200c07a5b70eff7 100644
--- a/pkg/compiler/lib/src/js_emitter/old_emitter/type_test_emitter.dart
+++ b/pkg/compiler/lib/src/js_emitter/old_emitter/type_test_emitter.dart
@@ -27,6 +27,9 @@ class TypeTestEmitter extends CodeEmitterHelper {
   Set<FunctionType> checkedNonGenericFunctionTypes =
       new Set<FunctionType>();
 
+  /// Initially contains all classes that need RTI. After
+  /// [computeNeededClasses]
+  /// this set only contains classes that are only used for RTI.
   final Set<ClassElement> rtiNeededClasses = new Set<ClassElement>();
 
   Iterable<ClassElement> cachedClassesUsingTypeVariableTests;