Fix some analyzer warnings and hints in pkg/watcher.

pkg/watcher/lib/src/constructable_file_system_event.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.constructable_file_system_event;
 
 import 'dart:io';
 
 abstract class _ConstructableFileSystemEvent implements FileSystemEvent {
   final bool isDirectory;
   final String path;
-  final int type;
+  int get type;
 
   _ConstructableFileSystemEvent(this.path, this.isDirectory);
 }
 
 class ConstructableFileSystemCreateEvent extends _ConstructableFileSystemEvent
     implements FileSystemCreateEvent {
   final type = FileSystemEvent.CREATE;
 
   ConstructableFileSystemCreateEvent(String path, bool isDirectory)
       : super(path, isDirectory);
@@ skipping 28 matching lines @@
     implements FileSystemMoveEvent {
   final String destination;
   final type = FileSystemEvent.MOVE;
 
   ConstructableFileSystemMoveEvent(String path, bool isDirectory,
           this.destination)
       : super(path, isDirectory);
 
   String toString() => "FileSystemMoveEvent('$path', '$destination')";
 }