Disable closure wrapping by default

lib/runtime/dart/_isolate_helper.js

Index: lib/runtime/dart/_isolate_helper.js
diff --git a/lib/runtime/dart/_isolate_helper.js b/lib/runtime/dart/_isolate_helper.js
index fc1cafccf353d663ff4d653e9603477f947f9bdb..7297fbcf81fca9b968a7fa08600e38a62a448eeb 100644
--- a/lib/runtime/dart/_isolate_helper.js
+++ b/lib/runtime/dart/_isolate_helper.js
@@ -952,9 +952,9 @@ var _isolate_helper;
     }
     static handleSpawnWorkerRequest(msg) {
       let replyPort = dart.dindex(msg, 'replyPort');
-      IsolateNatives.spawn(dart.as(dart.dindex(msg, 'functionName'), core.String), dart.as(dart.dindex(msg, 'uri'), core.String), dart.as(dart.dindex(msg, 'args'), core.List$(core.String)), dart.dindex(msg, 'msg'), false, dart.as(dart.dindex(msg, 'isSpawnUri'), core.bool), dart.as(dart.dindex(msg, 'startPaused'), core.bool)).then(dart.closureWrap((msg) => {
+      IsolateNatives.spawn(dart.as(dart.dindex(msg, 'functionName'), core.String), dart.as(dart.dindex(msg, 'uri'), core.String), dart.as(dart.dindex(msg, 'args'), core.List$(core.String)), dart.dindex(msg, 'msg'), false, dart.as(dart.dindex(msg, 'isSpawnUri'), core.bool), dart.as(dart.dindex(msg, 'startPaused'), core.bool)).then(dart.as((msg) => {
         dart.dinvoke(replyPort, 'send', msg);
-      }, "(List<dynamic>) → dynamic"), {
+      }, dart.throw_("Unimplemented type (List<dynamic>) → dynamic")), {
         onError: (errorMessage) => {
           dart.dinvoke(replyPort, 'send', new core.List.from([_SPAWN_FAILED_SIGNAL, errorMessage]));
         }