Allowing Window.onBeforeUnload event to work properly.

sdk/lib/_internal/compiler/implementation/lib/isolate_helper.dart

Index: sdk/lib/_internal/compiler/implementation/lib/isolate_helper.dart
diff --git a/sdk/lib/_internal/compiler/implementation/lib/isolate_helper.dart b/sdk/lib/_internal/compiler/implementation/lib/isolate_helper.dart
index 3016e78f87141b88058eb04cc9b49b89cfd11a08..f47100ec7a8ddfaeb48369ed0fcf5ea516f19d0a 100644
--- a/sdk/lib/_internal/compiler/implementation/lib/isolate_helper.dart
+++ b/sdk/lib/_internal/compiler/implementation/lib/isolate_helper.dart
@@ -54,9 +54,10 @@ class JsIsolateSink extends StreamSink<dynamic> implements IsolateSink {
  * Called by the compiler to support switching
  * between isolates when we get a callback from the DOM.
  */
-void _callInIsolate(_IsolateContext isolate, Function function) {
-  isolate.eval(function);
+_callInIsolate(_IsolateContext isolate, Function function) {
+  var result = isolate.eval(function);
   _globalState.topEventLoop.run();
+  return result;
 }
 
 /**