Rewrite file-system-watcher to better handle the different system APIs.

sdk/lib/io/file_system_entity.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.
 
 part of dart.io;
 
 class FileSystemEntityType {
   static const FILE = const FileSystemEntityType._internal(0);
   static const DIRECTORY = const FileSystemEntityType._internal(1);
   static const LINK = const FileSystemEntityType._internal(2);
@@ skipping 352 matching lines @@
    *   * The [Stream] is canceled, e.g. by calling `cancel` on the
    *      [StreamSubscription].
    *   * The [FileSystemEntity] being watches, is deleted.
    *
    * Use `events` to specify what events to listen for. The constants in
    * [FileSystemEvent] can be or'ed together to mix events. Default is
    * [FileSystemEvent.ALL].
    */
   Stream<FileSystemEvent> watch({int events: FileSystemEvent.ALL,
                                  bool recursive: false})
-     => new _FileSystemWatcher(_trimTrailingPathSeparators(path),
-                               events,
-                               recursive).stream;
+     => _FileSystemWatcher.watch(_trimTrailingPathSeparators(path),
+                                 events,
+                                 recursive);
 
   Future<FileSystemEntity> _delete({bool recursive: false});
   void _deleteSync({bool recursive: false});
 
   /**
    * Checks whether two paths refer to the same object in the
    * file system. Returns a [:Future<bool>:] that completes with the result.
    *
    * Comparing a link to its target returns false, as does comparing two links
    * that point to the same target.  To check the target of a link, use
@@ skipping 348 matching lines @@
   String toString() {
     var buffer = new StringBuffer();
     buffer.write("FileSystemMoveEvent('$path'");
     if (destination != null) buffer.write(", '$destination'");
     buffer.write(')');
     return buffer.toString();
   }
 }
 
 
-abstract class _FileSystemWatcher {
-  external factory _FileSystemWatcher(String path, int events, bool recursive);
+class _FileSystemWatcher {
+  external static Stream<FileSystemEvent> watch(
+      String path, int events, bool recursive);
   external static bool get isSupported;
-
-  Stream<FileSystemEvent> get stream;
 }