Refactor Future constructors.

tests/language/store_to_load_forwarding_phis_vm_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.
 // Test correct handling of phis with only environment uses that were inserted
 // by store to load forwarding.
 // VMOptions=--optimization_counter_threshold=100
 
 library store_to_load_forwarding_phis_vm_test;
 
 import 'dart:async';
 
 class A {
   var _foo;
 
   get foo {
     if (_foo == null) {
       _foo = new A();
     }
     return _foo;
   }
 }
 
 foo(obj) {
   var a = obj.foo;
-  return new Future.immediate(null).then((val) { });
+  return new Future.value().then((val) { });
 }
 
 main() {
   final obj = new A();
   for (var i = 0; i < 200; i++) {
     foo(obj);
   }
 }