Fall back to PollingDirectoryWatcher if FileSystemEntity:watch is not supported.

pkg/watcher/lib/src/directory_watcher.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.directory_watcher;
 
 import 'dart:async';
 import 'dart:io';
 
 import 'watch_event.dart';
@@ skipping 37 matching lines @@
   ///
   /// If a native directory watcher is available for this platform, this will
   /// use it. Otherwise, it will fall back to a [PollingDirectoryWatcher].
   ///
   /// If [_pollingDelay] is passed, it specifies the amount of time the watcher
   /// will pause between successive polls of the directory contents. Making this
   /// shorter will give more immediate feedback at the expense of doing more IO
   /// and higher CPU usage. Defaults to one second. Ignored for non-polling
   /// watchers.
   factory DirectoryWatcher(String directory, {Duration pollingDelay}) {
-    if (Platform.isLinux) return new LinuxDirectoryWatcher(directory);
-    if (Platform.isMacOS) return new MacOSDirectoryWatcher(directory);
-    if (Platform.isWindows) return new WindowsDirectoryWatcher(directory);
+    if (FileSystemEntity.isWatchSupported) {
+      if (Platform.isLinux) return new LinuxDirectoryWatcher(directory);
+      if (Platform.isMacOS) return new MacOSDirectoryWatcher(directory);
+      if (Platform.isWindows) return new WindowsDirectoryWatcher(directory);
+    }
     return new PollingDirectoryWatcher(directory, pollingDelay: pollingDelay);
   }
 }