Update unittest to new package layout.

pkg/unittest/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.
-
-/**
- * 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);
-
-/**
- * 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].
- */
-Matcher completion(matcher) => new _Completes(wrapMatcher(matcher));
-
-class _Completes extends BaseMatcher {
-  final Matcher _matcher;
-
-  const _Completes(this._matcher);
-
-  bool matches(item, MatchState matchState) {
-    if (item is! Future) return false;
-
-    item.onComplete(expectAsync1((future) {
-      var reason = 'Expected future to complete successfully, but it failed '
-        'with ${future.exception}';
-      if (future.stackTrace != null) {
-        var stackTrace = future.stackTrace.toString();
-        stackTrace = '  ${stackTrace.replaceAll('\n', '\n  ')}';
-        reason = '$reason\nStack trace:\n$stackTrace';
-      }
-
-      expect(future.hasValue, reason: reason);
-      if (_matcher != null) expect(future.value, _matcher);
-    }));
-
-    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;
-  }
-}