Wrap Directory.watch on Mac OS for the watcher package.

pkg/watcher/lib/src/utils.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 watcher.utils;
 
 import 'dart:async';
 import 'dart:io';
 import 'dart:collection';
 
 /// Returns `true` if [error] is a [FileSystemException] for a missing
 /// directory.
 bool isDirectoryNotFoundException(error) {
   if (error is! FileSystemException) return false;
 
   // See dartbug.com/12461 and tests/standalone/io/directory_error_test.dart.
   var notFoundCode = Platform.operatingSystem == "windows" ? 3 : 2;
   return error.osError.errorCode == notFoundCode;
 }
 
+/// Returns the union of all elements in each set in [sets].
+Set unionAll(Iterable<Set> sets) =>
+  sets.fold(new Set(), (union, set) => union.union(set));
+
 /// Returns a buffered stream that will emit the same values as the stream
 /// returned by [future] once [future] completes.
 ///
 /// If [future] completes to an error, the return value will emit that error and
 /// then close.
 Stream futureStream(Future<Stream> future) {
   var controller = new StreamController(sync: true);
   future.then((stream) {
     stream.listen(
         controller.add,
@@ skipping 33 matching lines @@
       });
     }, handleDone: (sink) {
       if (batch.isNotEmpty) {
         sink.add(batch.toList());
         batch.clear();
       }
       sink.close();
     }).bind(input);
   }
 }