dart2js: Fix some problems with mixins and generics

pkg/compiler/lib/src/js_backend/runtime_types.dart

Index: pkg/compiler/lib/src/js_backend/runtime_types.dart
diff --git a/pkg/compiler/lib/src/js_backend/runtime_types.dart b/pkg/compiler/lib/src/js_backend/runtime_types.dart
index 1d7470ffdf17a0cbad922d116f70a44a7ccef62c..21e3e76783e6fc70d4efea0ed572c19ea430d82e 100644
--- a/pkg/compiler/lib/src/js_backend/runtime_types.dart
+++ b/pkg/compiler/lib/src/js_backend/runtime_types.dart
@@ -490,6 +490,9 @@ class _RuntimeTypes implements RuntimeTypes {
       }
     }
     return instantiated..addAll(collector.classes);
+
+    // TODO(sra): This computation misses substitutions for reading type
+    // parameters.
   }
 
   @override