Write our JS blobs for handling native classes in Dart.

frog/leg/lib/js_helper.dart

Index: frog/leg/lib/js_helper.dart
===================================================================
--- frog/leg/lib/js_helper.dart	(revision 5677)
+++ frog/leg/lib/js_helper.dart	(working copy)
@@ -8,6 +8,7 @@
 
 #source('constant_map.dart');
 #source('date_helper.dart');
+#source('native_helper.dart');
 #source('regexp_helper.dart');
 #source('string_helper.dart');
 
@@ -1310,7 +1311,7 @@
   function = JS("var", @"""function() {
     return #(#, #, arguments.length, arguments[0], arguments[1]);
   }""",
-  invokeClosure,
+  JS_TO_CLOSURE(invokeClosure),
   closure,
   JS_CURRENT_ISOLATE());