Refactor Future constructors.

pkg/unittest/lib/unittest.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.
 
 /**
  * A library for writing dart unit tests.
  *
  * To import this library, install the
  * [unittest package](http://pub.dartlang.org/packages/unittest) via the pub
  * package manager. See the [Getting Started](http://pub.dartlang.org/doc)
@@ skipping 650 matching lines @@
 /**
  * Runs [callback] at the end of the event loop. Note that we don't wrap
  * the callback in guardAsync; this is for test framework functions which
  * should not be throwing unexpected exceptions that end up failing test
  * cases! Furthermore, we need the final exception to be thrown but not
  * caught by the test framework if any test cases failed. However, tests
  * that make use of a similar defer function *should* wrap the callback
  * (as we do in unitttest_test.dart).
  */
 _defer(void callback()) {
-  (new Future.immediate(null)).then((_) => callback());
+  (new Future.value()).then((_) => callback());
 }
 
 void rerunTests() {
   _uncaughtErrorMessage = null;
   _initialized = true; // We don't want to reset the test array.
   runTests();
 }
 
 /**
  * Filter the tests. [testFilter] can be a [RegExp], a [String] or a
@@ skipping 175 matching lines @@
 }
 
 /** Enable a test by ID. */
 void enableTest(int testId) => _setTestEnabledState(testId, true);
 
 /** Disable a test by ID. */
 void disableTest(int testId) => _setTestEnabledState(testId, false);
 
 /** Signature for a test function. */
 typedef dynamic TestFunction();