pkg/unittest: Moved `fail` to matcher library

pkg/unittest/lib/src/future_matchers.dart

 // Copyright (c) 2012, 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.
 
 part of matcher;
 
 /**
  * Matches a [Future] that completes successfully with a value. Note that this
  * creates an asynchronous expectation. The call to `expect()` that includes
  * this will return immediately and execution will continue. Later, when the
@@ skipping 27 matching lines @@
     item.then((value) {
       done(() { if (_matcher != null) expect(value, _matcher); });
     }, onError: (e) {
       var reason = 'Expected future to complete successfully, but it failed '
                    'with ${e.error}';
       if (e.stackTrace != null) {
         var stackTrace = e.stackTrace.toString();
         stackTrace = '  ${stackTrace.replaceAll('\n', '\n  ')}';
         reason = '$reason\nStack trace:\n$stackTrace';
       }
-      done(() => expect(false, isTrue, reason: reason));
+      done(() => fail(reason));
     });
 
     return true;
   }
 
   Description describe(Description description) {
     if (_matcher == null) {
       description.add('completes successfully');
     } else {
       description.add('completes to a value that ').addDescriptionOf(_matcher);
     }
     return description;
   }
 }