fix future onError handler wrapping to avoid cast error

sdk/lib/async/future_impl.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;
 
 /** The onValue and onError handlers return either a value or a future */
 typedef FutureOr<T> _FutureOnValue<S, T>(S value);
 /** Test used by [Future.catchError] to handle skip some errors. */
 typedef bool _FutureErrorTest(var error);
@@ skipping 219 matching lines @@
     _resultOrListeners = source;
   }
 
   Future<E> then<E>(
       FutureOr<E> f(T value), { Function onError }) {
     Zone currentZone = Zone.current;
     ZoneUnaryCallback registered;
     if (!identical(currentZone, _ROOT_ZONE)) {
       f = currentZone.registerUnaryCallback<FutureOr<E>, T>(f);
       if (onError != null) {
-        onError = _registerErrorHandler<T>(onError, currentZone);

[floitsch, 2017/02/27 13:19:04]

This was a mistake. It should have been `_registerErrorHandler<E>`.

[Jennifer Messerly, 2017/02/27 18:41:02]

okay. I can fix it that way, however: at least a couple of places in the SDK use
"void" as the return type of the error handler.

Are those locations correct? Should they return a value of some sort? I can
change the return type and return null.

[floitsch, 2017/02/27 19:02:39]

I will have a look tomorrow.
If you have these places handy, please send them to me. Otherwise I will grep
for them tomorrow myself.

+        onError = _registerErrorHandler(onError, currentZone);
       }
     }
     return _thenNoZoneRegistration<E>(f, onError);
   }
 
   // This method is used by async/await.
   Future<E> _thenNoZoneRegistration<E>(f(T value), Function onError) {
     _Future<E> result = new _Future<E>();
     _addListener(new _FutureListener<T, E>.then(result, f, onError));
     return result;
   }
 
   Future<T> catchError(Function onError, { bool test(error) }) {
     _Future<T> result = new _Future<T>();
     if (!identical(result._zone, _ROOT_ZONE)) {
-      onError = _registerErrorHandler<T>(onError, result._zone);
+      onError = _registerErrorHandler(onError, result._zone);
       if (test != null) test = result._zone.registerUnaryCallback(test);
     }
     _addListener(new _FutureListener<T, T>.catchError(
         result, onError, test));
     return result;
   }
 
   Future<T> whenComplete(action()) {
     _Future<T> result = new _Future<T>();
     if (!identical(result._zone, _ROOT_ZONE)) {
@@ skipping 470 matching lines @@
       }
     }, onError: (e, s) {
       if (timer.isActive) {
         timer.cancel();
         result._completeError(e, s);
       }
     });
     return result;
   }
 }