Mark the "completes" matcher as final.

pkg/scheduled_test/lib/src/scheduled_future_matchers.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 scheduled_future_matchers;
 
 import 'dart:async';
 
 import 'package:stack_trace/stack_trace.dart';
 
 import '../scheduled_test.dart';
 
 /// 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].
 ///
 /// This differs from the `completes` matcher in `unittest` in that it pipes any
 /// errors in the Future to [currentSchedule], rather than reporting them in the
 /// [expect]'s error message.
-Matcher completes = const _ScheduledCompletes(null, null);
+final Matcher completes = const _ScheduledCompletes(null, 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].
 ///
 /// [description] is an optional tag that can be used to identify the completion
@@ skipping 42 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;
   }
 }