Add FileSystemWatcher class to dart:io.

sdk/lib/io/directory_impl.dart

Index: sdk/lib/io/directory_impl.dart
diff --git a/sdk/lib/io/directory_impl.dart b/sdk/lib/io/directory_impl.dart
index 39d36cfe22c5997a49c2a49a3a76e82c56a152ec..c9eec06376a5df378adebb60dba65149c2a2bbb8 100644
--- a/sdk/lib/io/directory_impl.dart
+++ b/sdk/lib/io/directory_impl.dart
@@ -4,7 +4,7 @@
 
 part of dart.io;
 
-class _Directory implements Directory {
+class _Directory extends FileSystemEntity implements Directory {
   static const CREATE_REQUEST = 0;
   static const DELETE_REQUEST = 1;
   static const EXISTS_REQUEST = 2;
@@ -229,35 +229,22 @@ class _Directory implements Directory {
     return new Directory(newPath);
   }
 
-  static String _trimTrailingPathSeparators(String path) {
-    // Don't handle argument errors here.
-    if (path is! String) return path;
-    if (Platform.operatingSystem == 'windows') {
-      while (path.length > 1 &&
-             (path.endsWith(Platform.pathSeparator) ||
-              path.endsWith('/'))) {
-        path = path.substring(0, path.length - 1);
-      }
-    } else {
-      while (path.length > 1 && path.endsWith(Platform.pathSeparator)) {
-        path = path.substring(0, path.length - 1);
-      }
-    }
-    return path;
-  }
-
   Stream<FileSystemEntity> list({bool recursive: false,
                                  bool followLinks: true}) {
-    return new _AsyncDirectoryLister(_trimTrailingPathSeparators(path),
-                                     recursive,
-                                     followLinks).stream;
+    return new _AsyncDirectoryLister(
+        FileSystemEntity._trimTrailingPathSeparators(path),
+        recursive,
+        followLinks).stream;
   }
 
   List listSync({bool recursive: false, bool followLinks: true}) {
     if (recursive is! bool || followLinks is! bool) {
       throw new ArgumentError();
     }
-    return _list(_trimTrailingPathSeparators(path), recursive, followLinks);
+    return _list(
+        FileSystemEntity._trimTrailingPathSeparators(path),
+        recursive,
+        followLinks);
   }
 
   String toString() => "Directory: '$path'";