Avoid relying on fuzzy arrow for arity check in _FutureListener

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(Object error);
@@ skipping 118 matching lines @@
     return _zone.runUnary<FutureOr<T>, S>(_onValue, sourceResult);
   }
 
   bool matchesErrorTest(AsyncError asyncError) {
     if (!hasErrorTest) return true;
     return _zone.runUnary<bool, Object>(_errorTest, asyncError.error);
   }
 
   FutureOr<T> handleError(AsyncError asyncError) {
     assert(handlesError && hasErrorCallback);
-    if (errorCallback is ZoneBinaryCallback) {
+    if (errorCallback is ZoneBinaryCallback<dynamic, Null, Null>) {

[floitsch, 2017/08/31 07:54:52]

Might as well use the correct type (see 2 lines below).

       var typedErrorCallback = errorCallback as Object
           /*=ZoneBinaryCallback<FutureOr<T>, Object, StackTrace>*/;
       return _zone.runBinary(
           typedErrorCallback, asyncError.error, asyncError.stackTrace);
     } else {
       return _zone.runUnary<FutureOr<T>, Object>(
           errorCallback, asyncError.error);
     }
   }
 
@@ skipping 585 matching lines @@
       }
     }, onError: (e, s) {
       if (timer.isActive) {
         timer.cancel();
         result._completeError(e, s);
       }
     });
     return result;
   }
 }