Refactor Future constructors.

pkg/scheduled_test/test/descriptor/async_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 'dart:async';
 import 'dart:io';
 
 import 'package:pathos/path.dart' as path;
 import 'package:scheduled_test/descriptor.dart' as d;
 import 'package:scheduled_test/scheduled_test.dart';
@@ skipping 108 matching lines @@
       expect(errors.length, equals(1));
       expect(errors.first.error,
           matches(r"^Directory not found: '[^']+[\\/]dir'\.$"));
     });
   }, passing: ['test 2']);
 
   expectTestsPass("async().load() fails", () {
     test('test', () {
       scheduleSandbox();
 
-      expect(d.async(new Future.immediate(d.file('name.txt')))
+      expect(d.async(new Future.value(d.file('name.txt')))
               .load('path').toList(),
           throwsA(equals("AsyncDescriptors don't support load().")));
     });
   });
 
   expectTestsPass("async().read() fails", () {
     test('test', () {
       scheduleSandbox();
 
-      expect(d.async(new Future.immediate(d.file('name.txt'))).read().toList(),
+      expect(d.async(new Future.value(d.file('name.txt'))).read().toList(),
           throwsA(equals("AsyncDescriptors don't support read().")));
     });
   });
 }