Implement JsMethodMirror.parameters.

dart/sdk/lib/_internal/lib/js_rti.dart

Index: dart/sdk/lib/_internal/lib/js_rti.dart
diff --git a/dart/sdk/lib/_internal/lib/js_rti.dart b/dart/sdk/lib/_internal/lib/js_rti.dart
index 01d8aceb40e21329c9adc14a41a8c425d8b60e82..4441214b8e9de4386f3a08042c4de780eca502f0 100644
--- a/dart/sdk/lib/_internal/lib/js_rti.dart
+++ b/dart/sdk/lib/_internal/lib/js_rti.dart
@@ -132,9 +132,11 @@ String runtimeTypeToString(var type) {
   } else if (isJsArray(type)) {
     // A list representing a type with arguments.
     return getRuntimeTypeAsString(type);
-  } else {
+  } else if (isJsFunction(type)) {
     // A reference to the constructor.
     return getConstructorName(type);
+  } else {
+    return null;
   }
 }