Support length operation without opening file.

runtime/bin/file.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.
 
 
 /**
  * FileMode describes the modes in which a file can be opened.
  */
 class FileMode {
   static final READ = const FileMode._internal(0);
@@ skipping 65 matching lines @@
    */
   void directory(void callback(Directory dir));
 
   /**
    * Synchronously get a Directory object for the directory containing
    * this file.
    */
   Directory directorySync();
 
   /**
+   * Get the length of the file. When the operation completes the
+   * callback is called with the length.
+   */
+  void length(void callback(int length));
+
+  /**
+   * Synchronously get the length of the file.
+   */
+  int lengthSync();
+
+  /**
    * Open the file for random access operations. When the file is
    * opened the callback is called with the resulting
    * RandomAccessFile. RandomAccessFiles must be closed using the
    * [close] method. If the file cannot be opened [onError] is called.
    *
    * Files can be opened in three modes:
    *
    * FileMode.READ: open the file for reading. If the file does not
    * exist [onError] is called.
    *
@@ skipping 283 matching lines @@
         sb.add(" ($osError)");
       }
     } else if (osError != null) {
       sb.add(": osError");
     }
     return sb.toString();
   }
   final String message;
   final OSError osError;
 }