dart2js: don't reify type variables for reflectable typedefs.

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

Index: pkg/compiler/lib/src/js_emitter/metadata_collector.dart
diff --git a/pkg/compiler/lib/src/js_emitter/metadata_collector.dart b/pkg/compiler/lib/src/js_emitter/metadata_collector.dart
index 48df1396d7382af53e862e07d1ea95a544a56915..bd4ab2977a33f45cc8051c6ee5843a4bb8377247 100644
--- a/pkg/compiler/lib/src/js_emitter/metadata_collector.dart
+++ b/pkg/compiler/lib/src/js_emitter/metadata_collector.dart
@@ -91,16 +91,19 @@ class MetadataCollector {
             _emitter.constantReference(constant.value), _compiler).getText());
   }
 
-  int reifyType(DartType type) {
+  int reifyType(DartType type, {bool ignoreTypeVariables: false}) {
     return reifyTypeForOutputUnit(type,
-                                  _compiler.deferredLoadTask.mainOutputUnit);
+                                  _compiler.deferredLoadTask.mainOutputUnit,
+                                  ignoreTypeVariables: ignoreTypeVariables);
   }
 
-  int reifyTypeForOutputUnit(DartType type, OutputUnit outputUnit) {
+  int reifyTypeForOutputUnit(DartType type, OutputUnit outputUnit,
+                             {bool ignoreTypeVariables: false}) {
     jsAst.Expression representation =
         _backend.rti.getTypeRepresentation(
             type,
             (variable) {
+              if (ignoreTypeVariables) return new jsAst.LiteralNull();
               return js.number(
                   _typeVariableHandler.reifyTypeVariable(
                       variable.element));