fix spelling in core libraries

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;
 
 /**
  * The type of an entity on the file system, such as a file, directory, or link.
  *
  * These constants are used by the [FileSystemEntity] class
@@ skipping 798 matching lines @@
 
   String toString() => "FileSystemDeleteEvent('$path')";
 }
 
 
 /**
  * File system event for moving of file system objects.
  */
 class FileSystemMoveEvent extends FileSystemEvent {
   /**
-   * If the underlaying implementation is able to identify the destination of
+   * If the underlying implementation is able to identify the destination of
    * the moved file, [destination] will be set. Otherwise, it will be `null`.
    */
   final String destination;
 
   FileSystemMoveEvent._(path, isDirectory, this.destination)
       : super._(FileSystemEvent.MOVE, path, isDirectory);
 
   String toString() {
     var buffer = new StringBuffer();
     buffer.write("FileSystemMoveEvent('$path'");
     if (destination != null) buffer.write(", '$destination'");
     buffer.write(')');
     return buffer.toString();
   }
 }
 
 
 class _FileSystemWatcher {
   external static Stream<FileSystemEvent> _watch(
       String path, int events, bool recursive);
   external static bool get isSupported;
 }