fix future onError handler wrapping to avoid cast error

sdk/lib/async/future_impl.dart

Index: sdk/lib/async/future_impl.dart
diff --git a/sdk/lib/async/future_impl.dart b/sdk/lib/async/future_impl.dart
index eaad3b48c2e8d97ef1691845eeb8fecb26a1d3cb..0fd18eae6a97af2ae3f8513007455d4d6db04c7e 100644
--- a/sdk/lib/async/future_impl.dart
+++ b/sdk/lib/async/future_impl.dart
@@ -237,7 +237,7 @@ class _Future<T> implements Future<T> {
     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);
@@ -253,7 +253,7 @@ class _Future<T> implements Future<T> {
   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(