Add second argument to Future error handlers.

tests/lib/async/stack_trace12_test.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.
 
 import 'package:expect/expect.dart';
 import 'package:async_helper/async_helper.dart';
 import 'dart:async';
 
 main() {
-  Expect.identical(Zone.ROOT, Zone.current);
-  Zone forked = Zone.current.fork();
-  var f = Zone.current.bindCallback(() {
-    Expect.identical(Zone.ROOT, Zone.current);
-  }, runGuarded: false);
-  forked.run(() {
-    f();
-  });
-}
+  StackTrace trace;
+  asyncStart();
+  var f = new Future(() { throw "foo"; });
+  f.then((_) { throw "Unreachable"; },
+    onError: (e, st) {
+      Expect.equals("foo", e);
+      Expect.isNotNull(st);
+      asyncEnd();
+    });
+}