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

tool/input_sdk/private/isolate_helper.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library dart._isolate_helper;
 
 import 'dart:_js_embedded_names' show
     CLASS_ID_EXTRACTOR,
     CLASS_FIELDS_EXTRACTOR,
     CURRENT_SCRIPT,
@@ skipping 738 matching lines @@
   static String computeThisScript() {
     // See: https://github.com/dart-lang/dev_compiler/issues/164
     // var currentScript = JS_EMBEDDED_GLOBAL('', CURRENT_SCRIPT);
     var currentScript = JS('var', 'document.currentScript');
     if (currentScript != null) {
       return JS('String', 'String(#.src)', currentScript);
     }
     if (Primitives.isD8) return computeThisScriptD8();
     if (Primitives.isJsshell) return computeThisScriptJsshell();
     // A worker has no script tag - so get an url from a stack-trace.
-    if (_globalState.isWorker) return computeThisScriptFromTrace();
+    if (_globalState != null && _globalState.isWorker) {
+      return computeThisScriptFromTrace();
+    }
     return null;
   }
 
   static String computeThisScriptJsshell() {
     return JS('String|Null', 'thisFilename()');
   }
 
   // TODO(ahe): The following is for supporting D8.  We should move this code
   // to a helper library that is only loaded when testing on D8.
   static String computeThisScriptD8() => computeThisScriptFromTrace();
@@ skipping 669 matching lines @@
   }
 
   bool operator==(Object other) {
     if (identical(other, this)) return true;
     if (other is CapabilityImpl) {
       return identical(_id, other._id);
     }
     return false;
   }
 }