Add support for file locking

runtime/bin/builtin_natives.cc

 // 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.
 
 #include <stdlib.h>
 #include <stdio.h>
 #include <string.h>
 
 #include "include/dart_api.h"
 
@@ skipping 32 matching lines @@
   V(File_ReadInto, 4)                                                          \
   V(File_WriteFrom, 4)                                                         \
   V(File_Position, 1)                                                          \
   V(File_SetPosition, 2)                                                       \
   V(File_Truncate, 2)                                                          \
   V(File_Length, 1)                                                            \
   V(File_LengthFromPath, 1)                                                    \
   V(File_Stat, 1)                                                              \
   V(File_LastModified, 1)                                                      \
   V(File_Flush, 1)                                                             \
+  V(File_Lock, 4)                                                              \
   V(File_Create, 1)                                                            \
   V(File_CreateLink, 2)                                                        \
   V(File_LinkTarget, 1)                                                        \
   V(File_Delete, 1)                                                            \
   V(File_DeleteLink, 1)                                                        \
   V(File_Rename, 2)                                                            \
   V(File_Copy, 2)                                                              \
   V(File_RenameLink, 2)                                                        \
   V(File_ResolveSymbolicLinks, 1)                                              \
   V(File_OpenStdio, 1)                                                         \
@@ skipping 69 matching lines @@
   } else {
     // Uses fwrite to support printing NUL bytes.
     fwrite(chars, 1, length, stdout);
     fputs("\n", stdout);
   }
   fflush(stdout);
 }
 
 }  // namespace bin
 }  // namespace dart