Fix type mismatch in File.fullPathSync.

runtime/bin/file_impl.dart

 // Copyright (c) 2011, 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.
 
 class _FileInputStream implements FileInputStream {
   _FileInputStream(File file) {
     _file = new File(file.name);
     _file.openSync();
   }
 
@@ skipping 395 matching lines @@
   static int _readList(int id, List<int> buffer, int offset, int bytes)
       native "File_ReadList";
   static int _writeByte(int id, int value) native "File_WriteByte";
   static int _writeList(int id, List<int> buffer, int offset, int bytes)
       native "File_WriteList";
   static int _writeString(int id, String string) native "File_WriteString";
   static int _position(int id) native "File_Position";
   static int _length(int id) native "File_Length";
   static int _flush(int id) native "File_Flush";
   static bool _create(String name) native "File_Create";
-  static int _fullPath(String name) native "File_FullPath";
+  static String _fullPath(String name) native "File_FullPath";
 
   static int _checkReadWriteListArguments(int length, int offset, int bytes) {
     if (offset < 0) return offset;
     if (bytes < 0) return bytes;
     if ((offset + bytes) > length) return offset + bytes;
     return 0;
   }
 
   void exists() {
     _asyncUsed = true;
@@ skipping 423 matching lines @@
   var _closeHandler;
   var _readByteHandler;
   var _readListHandler;
   var _noPendingWriteHandler;
   var _positionHandler;
   var _lengthHandler;
   var _flushHandler;
   var _fullPathHandler;
   var _errorHandler;
 }