fixes for angular hello: String + and disable broken arg parsing

lib/runtime/dart/_isolate_helper.js

 var _isolate_helper;
 (function(exports) {
   'use strict';
   // Function _serializeMessage: (dynamic) → dynamic
   function _serializeMessage(message) {
     return new _Serializer().serialize(message);
   }
   // Function _deserializeMessage: (dynamic) → dynamic
   function _deserializeMessage(message) {
     return new _Deserializer().deserialize(message);
@@ skipping 427 matching lines @@
       this.managers = new (core.Map$(core.int, dart.dynamic))();
       if (this.isWorker) {
         this.mainManager = new _MainManagerStub();
         this[_nativeInitWorkerMessageHandler]();
       }
     }
     [_nativeDetectEnvironment]() {
       let isWindowDefined = exports.globalWindow != null;
       let isWorkerDefined = exports.globalWorker != null;
       this.isWorker = !dart.notNull(isWindowDefined) && dart.notNull(exports.globalPostMessageDefined);
-      this.supportsWorkers = dart.notNull(this.isWorker) || dart.notNull(isWorkerDefined) && dart.notNull(IsolateNatives.thisScript != null);
+      this.supportsWorkers = dart.notNull(this.isWorker) || dart.notNull(isWorkerDefined) && IsolateNatives.thisScript != null;
       this.fromCommandLine = !dart.notNull(isWindowDefined) && !dart.notNull(this.isWorker);
     }
     [_nativeInitWorkerMessageHandler]() {
       let function$0 = function(f, a) {
         return function(e) {
           f(a, e);
         };
       }(_foreign_helper.DART_CLOSURE_TO_JS(IsolateNatives[_processWorkerMessage]), this.mainManager);
       self.onmessage = function$0;
       self.dartPrint = self.dartPrint || function(serialize) {
@@ skipping 545 matching lines @@
       let isSpawnUri = false;
       return IsolateNatives.spawn(name, null, null, message, isLight, isSpawnUri, startPaused);
     }
     static spawnUri(uri, args, message, startPaused) {
       IsolateNatives.enableSpawnWorker = true;
       let isLight = false;
       let isSpawnUri = true;
       return IsolateNatives.spawn(null, uri.toString(), args, message, isLight, isSpawnUri, startPaused);
     }
     static spawn(functionName, uri, args, message, isLight, isSpawnUri, startPaused) {
-      if (dart.notNull(uri != null) && dart.notNull(uri.endsWith(".dart"))) {
-        uri = core.String['+'](uri, ".js");
+      if (uri != null && dart.notNull(uri.endsWith(".dart"))) {
+        uri = dart.notNull(uri) + ".js";
       }
       let port = new isolate.ReceivePort();
       let completer = dart.as(new async.Completer(), async.Completer$(core.List));
       port.first.then(msg => {
         if (dart.equals(dart.dindex(msg, 0), _SPAWNED_SIGNAL)) {
           completer.complete(msg);
         } else {
           dart.assert(dart.equals(dart.dindex(msg, 0), _SPAWN_FAILED_SIGNAL));
           completer.completeError(dart.dindex(msg, 1));
         }
@@ skipping 366 matching lines @@
   exports.leaveJsAsync = leaveJsAsync;
   exports.isWorker = isWorker;
   exports.startRootIsolate = startRootIsolate;
   exports.IsolateNatives = IsolateNatives;
   exports.RawReceivePortImpl = RawReceivePortImpl;
   exports.ReceivePortImpl = ReceivePortImpl;
   exports.TimerImpl = TimerImpl;
   exports.hasTimer = hasTimer;
   exports.CapabilityImpl = CapabilityImpl;
 })(_isolate_helper || (_isolate_helper = {}));