fixed one-liners for file and directory

sdk/lib/io/directory.dart

 // Copyright (c) 2012, 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;
 
 /**
- * [Directory] objects are used for working with directories.
+ * A reference to a directory (or _folder_) on the file system.
  */
 abstract class Directory implements FileSystemEntity {
   /**
    * Creates a directory object. The path is either an absolute path,
    * or it is a relative path which is interpreted relative to the directory
    * in which the Dart VM was started.
    */
   factory Directory(String path) => new _Directory(path);
 
   /**
@@ skipping 176 matching lines @@
       if (path != null) {
         sb.write(", path = $path");
       }
     }
     return sb.toString();
   }
   final String message;
   final String path;
   final OSError osError;
 }