Loosen type check on runZoned errorHandler

sdk/lib/async/zone.dart

 // Copyright (c) 2013, 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;
 
 typedef R ZoneCallback<R>();
 typedef R ZoneUnaryCallback<R, T>(T arg);
 typedef R ZoneBinaryCallback<R, T1, T2>(T1 arg1, T2 arg2);
 
@@ skipping 1420 matching lines @@
  *       new Future(() { throw "asynchronous error"; });
  *     }, onError: print);  // Will print "asynchronous error".
  */
 R runZoned<R>(R body(),
     {Map zoneValues, ZoneSpecification zoneSpecification, Function onError}) {
   HandleUncaughtErrorHandler errorHandler;
   if (onError != null) {
     errorHandler = (Zone self, ZoneDelegate parent, Zone zone, error,
         StackTrace stackTrace) {
       try {
-        if (onError is ZoneBinaryCallback<R, Object, StackTrace>) {

[vsm, 2017/05/03 01:35:48]

We're failing this is-check now when onError is a (Object, [StackTrace]) =>
Void.  That's causing us to fall into the runUnary path and report suboptimal
errors on internal tests.

[Lasse Reichstein Nielsen, 2017/05/03 05:55:26]

This is another case where `void` as a type parameter would more precisely fix
the problem. We should make a list of these so we can fix them back when that
becomes an option.

[floitsch, 2017/05/03 09:48:11]

Agreed with Lasse: There should at the very least be a TODO. Look at the other
TODOs in the file and add a similar one.

[vsm, 2017/05/03 12:29:56]

Done.

+        if (onError is ZoneBinaryCallback<dynamic, Object, StackTrace>) {
           return self.parent.runBinary(onError, error, stackTrace);
         }
         return self.parent.runUnary(onError, error);
       } catch (e, s) {
         if (identical(e, error)) {
           return parent.handleUncaughtError(zone, error, stackTrace);
         } else {
           return parent.handleUncaughtError(zone, e, s);
         }
       }
     };
   }
   if (zoneSpecification == null) {
     zoneSpecification =
         new ZoneSpecification(handleUncaughtError: errorHandler);
   } else if (errorHandler != null) {
     zoneSpecification = new ZoneSpecification.from(zoneSpecification,
         handleUncaughtError: errorHandler);
   }
   Zone zone = Zone.current
       .fork(specification: zoneSpecification, zoneValues: zoneValues);
   if (onError != null) {
     return zone.runGuarded(body);
   } else {
     return zone.run(body);
   }
 }