Use stream matchers to unflake the mac OS watcher tests.

pkg/watcher/test/directory_watcher/linux_test.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.
 
 import 'package:scheduled_test/scheduled_test.dart';
 import 'package:watcher/src/directory_watcher/linux.dart';
 import 'package:watcher/watcher.dart';
 
 import 'shared.dart';
 import '../utils.dart';
@@ skipping 14 matching lines @@
 
   test('emits events for many nested files moved out then immediately back in',
       () {
     withPermutations((i, j, k) =>
         writeFile("dir/sub/sub-$i/sub-$j/file-$k.txt"));
     startWatcher(dir: "dir");
 
     renameDir("dir/sub", "sub");
     renameDir("sub", "dir/sub");
 
-    inAnyOrder(() {
-      withPermutations((i, j, k) =>
-          expectRemoveEvent("dir/sub/sub-$i/sub-$j/file-$k.txt"));
-    });
+    allowEither(() {
+      inAnyOrder(withPermutations((i, j, k) =>
+          isRemoveEvent("dir/sub/sub-$i/sub-$j/file-$k.txt")));
 
-    inAnyOrder(() {
-      withPermutations((i, j, k) =>
-          expectAddEvent("dir/sub/sub-$i/sub-$j/file-$k.txt"));
+      inAnyOrder(withPermutations((i, j, k) =>
+          isAddEvent("dir/sub/sub-$i/sub-$j/file-$k.txt")));
+    }, () {
+      inAnyOrder(withPermutations((i, j, k) =>
+          isModifyEvent("dir/sub/sub-$i/sub-$j/file-$k.txt")));
     });
   });
 }