Use a _FutureListener separate from the _Future to hold listeners.

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 dynamic ZoneCallback();
 typedef dynamic ZoneUnaryCallback(arg);
 typedef dynamic ZoneBinaryCallback(arg1, arg2);
 
@@ skipping 589 matching lines @@
   _ZoneFunction get _createTimer;
   _ZoneFunction get _createPeriodicTimer;
   _ZoneFunction get _print;
   _ZoneFunction get _fork;
   _ZoneFunction get _handleUncaughtError;
   _Zone get parent;
   _ZoneDelegate get _delegate;
   Map get _map;
 
   bool inSameErrorZone(Zone otherZone) {
-    return identical(errorZone, otherZone.errorZone);
+    return identical(this, otherZone) ||
+           identical(errorZone, otherZone.errorZone);
   }
 }
 
 class _CustomZone extends _Zone {
   // The actual zone and implementation of each of these
   // inheritable zone functions.
   _ZoneFunction _runUnary;
   _ZoneFunction _run;
   _ZoneFunction _runBinary;
   _ZoneFunction _registerCallback;
@@ skipping 312 matching lines @@
 ZoneBinaryCallback _rootRegisterBinaryCallback(
     Zone self, ZoneDelegate parent, Zone zone, f(arg1, arg2)) {
   return f;
 }
 
 AsyncError _rootErrorCallback(Zone self, ZoneDelegate parent, Zone zone,
                               Object error, StackTrace stackTrace) => null;
 
 void _rootScheduleMicrotask(Zone self, ZoneDelegate parent, Zone zone, f()) {
   if (!identical(_ROOT_ZONE, zone)) {
-    f = zone.bindCallback(f);
+    bool hasErrorHandler = !_ROOT_ZONE.inSameErrorZone(zone);
+    f = zone.bindCallback(f, runGuarded: hasErrorHandler);
   }
   _scheduleAsyncCallback(f);
 }
 
 Timer _rootCreateTimer(Zone self, ZoneDelegate parent, Zone zone,
                        Duration duration, void callback()) {
   if (!identical(_ROOT_ZONE, zone)) {
     callback = zone.bindCallback(callback);
   }
   return Timer._createTimer(duration, callback);
@@ skipping 288 matching lines @@
                                    handleUncaughtError: errorHandler);
   }
   Zone zone = Zone.current.fork(specification: zoneSpecification,
                                 zoneValues: zoneValues);
   if (onError != null) {
     return zone.runGuarded(body);
   } else {
     return zone.run(body);
   }
 }