Mark the "completes" matcher as final.

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
  * future completes, the actual expectation will run.
  *
  * To test that a Future completes with an exception, you can use [throws] and
  * [throwsA].
  */
-Matcher completes = const _Completes(null, '');
+final Matcher completes = const _Completes(null, '');
 
 /**
  * Matches a [Future] that completes succesfully with a value that matches
  * [matcher]. Note that this creates an asynchronous expectation. The call to
  * `expect()` that includes this will return immediately and execution will
  * continue. Later, when the future completes, the actual expectation will run.
  *
  * To test that a Future completes with an exception, you can use [throws] and
  * [throwsA].
  *
@@ skipping 33 matching lines @@
 
   Description describe(Description description) {
     if (_matcher == null) {
       description.add('completes successfully');
     } else {
       description.add('completes to a value that ').addDescriptionOf(_matcher);
     }
     return description;
   }
 }