pkg/matcher: refactored code around data-structure vs execution-based matchers

pkg/matcher/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.
 
 library matcher.future_matchers;
 
 import 'dart:async';
 
-import 'core_matchers.dart';
 import 'expect.dart';
 import 'interfaces.dart';
+import 'util.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].
 final Matcher completes = const _Completes(null, '');
 
@@ skipping 39 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;
   }
 }