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

tool/input_sdk/private/isolate_helper.dart

Index: tool/input_sdk/private/isolate_helper.dart
diff --git a/tool/input_sdk/private/isolate_helper.dart b/tool/input_sdk/private/isolate_helper.dart
index c0d7367acc296a730c307fa714ff9b2505afdb5e..b3e6635483d6a445242207e27e73b89b536f7532 100644
--- a/tool/input_sdk/private/isolate_helper.dart
+++ b/tool/input_sdk/private/isolate_helper.dart
@@ -756,7 +756,9 @@ class IsolateNatives {
     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;
   }