Use generic functions in zones.

tests/lib/async/zone_error_callback_test.dart

 // Copyright (c) 2014, 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() {
   asyncStart();
@@ skipping 14 matching lines @@
 class SomeError implements Error {
   final int id;
   const SomeError(this.id);
   StackTrace get stackTrace => stack2;
   String toString() => "SomeError($id)";
 }
 
 const error1 = const SomeError(1);
 const error2 = const SomeError(2);
 
-void expectError(e, s) {
+Null expectError(e, s) {
   // Remember one asyncStart per use of this callback.
   Expect.identical(error1, e);
   Expect.identical(stack1, s);
   asyncEnd();
+  return null;
 }
 
-void expectErrorOnly(e, s) {
+Null expectErrorOnly(e, s) {
   // Remember one asyncStart per use of this callback.
   Expect.identical(error1, e);
   asyncEnd();
+  return null;
 }
 
 AsyncError replace(self, parent, zone, e, s) {
   if (e == "ignore") return null; // For testing handleError throwing.
   Expect.identical(error1, e); // Ensure replacement only called once
   return new AsyncError(error2, stack2);
 }
 
 var replaceZoneSpec = new ZoneSpecification(errorCallback: replace);
 
 // Expectation after replacing.
-void expectReplaced(e, s) {
+Null expectReplaced(e, s) {
   Expect.identical(error2, e);
   Expect.identical(stack2, s);
   asyncEnd();
+  return null;
 }
 
 void testProgrammaticErrors(expectError) {
   {
     asyncStart();
     Completer c = new Completer();
     c.future.catchError(expectError);
     c.completeError(error1, stack1);
   }
 
@@ skipping 261 matching lines @@
     runZoned(testReplaced, zoneSpecification: replaceZoneSpec);
   }, zoneSpecification: replaceZoneSpec);
 
   // Use delegation to parent which replaces.
   runZoned(() {
     runZoned(testReplaced,
         zoneSpecification: new ZoneSpecification(
             errorCallback: (s, p, z, e, t) => p.errorCallback(z, e, t)));
   }, zoneSpecification: replaceZoneSpec);
 }