Refactor Future constructors.

pkg/scheduled_test/test/substitute_future_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.
 
 library substitute_future_test;
 
 import 'dart:async';
 
 import 'package:scheduled_test/src/substitute_future.dart';
 import 'package:unittest/unittest.dart';
@@ skipping 130 matching lines @@
       completer1.completeError(1);
       completer2.completeError(2);
       completer3.completeError(3);
     });
   });
 
   test('substituting after a future has completed is an error', () {
     var completer = new Completer();
     var future = new SubstituteFuture(completer.future);
     completer.complete('success');
-    expect(() => future.substitute(new Future.immediate(null)),
+    expect(() => future.substitute(new Future.value()),
         throwsStateError);
   });
 }