fix future onError handler wrapping to avoid cast error

sdk/lib/async/async_error.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.
 
 part of dart.async;
 
 _invokeErrorHandler(Function errorHandler,
                     Object error, StackTrace stackTrace) {
   if (errorHandler is ZoneBinaryCallback) {
     return errorHandler(error, stackTrace);
   } else {
     ZoneUnaryCallback unaryErrorHandler = errorHandler;
     return unaryErrorHandler(error);
   }
 }
 
-Function _registerErrorHandler<R>(Function errorHandler, Zone zone) {
+Function _registerErrorHandler(Function errorHandler, Zone zone) {
+  // We are not allowed to use 'void' as type argument for the generic type,
+  // so we use 'dynamic' instead.
   if (errorHandler is ZoneBinaryCallback) {
-    return zone.registerBinaryCallback<R, dynamic, StackTrace>(
-        errorHandler as dynamic/*=ZoneBinaryCallback<R, dynamic, StackTrace>*/);
+    return zone.registerBinaryCallback<dynamic, dynamic, StackTrace>(
+        errorHandler as dynamic/*=ZoneBinaryCallback<dynamic, dynamic, StackTrace>*/);
   } else {
-    return zone.registerUnaryCallback<R, dynamic>(
-        errorHandler as dynamic/*=ZoneUnaryCallback<R, dynamic>*/);
+    return zone.registerUnaryCallback<dynamic, dynamic>(
+        errorHandler as dynamic/*=ZoneUnaryCallback<dynamic, dynamic>*/);
   }
 }
 
 class _UncaughtAsyncError extends AsyncError {
   _UncaughtAsyncError(error, StackTrace stackTrace)
       : super(error, _getBestStackTrace(error, stackTrace));
 
   static StackTrace _getBestStackTrace(error, StackTrace stackTrace) {
     if (stackTrace != null) return stackTrace;
     if (error is Error) {
       return error.stackTrace;
     }
     return null;
   }
 
   String toString() {
     String result = "Uncaught Error: ${error}";
 
     if (stackTrace != null) {
       result += "\nStack Trace:\n$stackTrace";
     }
     return result;
   }
 }