fixes #221, initial sync*, async, async* implementation

lib/runtime/dart/_isolate_helper.js

 dart_library.library('dart/_isolate_helper', null, /* Imports */[
   "dart_runtime/dart",
   'dart/core',
   'dart/_interceptors',
   'dart/_js_helper',
   'dart/isolate',
   'dart/_foreign_helper',
   'dart/collection',
   'dart/async'
 ], /* Lazy imports */[
@@ skipping 932 matching lines @@
   class IsolateNatives extends core.Object {
     static computeThisScript() {
       let currentScript = document.currentScript;
       if (currentScript != null) {
         return String(currentScript.src);
       }
       if (dart.notNull(_js_helper.Primitives.isD8))
         return IsolateNatives.computeThisScriptD8();
       if (dart.notNull(_js_helper.Primitives.isJsshell))
         return IsolateNatives.computeThisScriptJsshell();
-      if (dart.notNull(exports._globalState.isWorker))
+      if (exports._globalState != null && dart.notNull(exports._globalState.isWorker)) {
         return IsolateNatives.computeThisScriptFromTrace();
+      }
       return null;
     }
     static computeThisScriptJsshell() {
       return dart.as(thisFilename(), core.String);
     }
     static computeThisScriptD8() {
       return IsolateNatives.computeThisScriptFromTrace();
     }
     static computeThisScriptFromTrace() {
       let stack = new Error().stack;
@@ skipping 589 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;
 });