Implement setPosition on File to seek to a byte position.

runtime/bin/builtin_in.cc

 // 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.
 
 #include <stdlib.h>
 #include <string.h>
 
 #include "include/dart_api.h"
 
 #include "bin/builtin.h"
@@ skipping 20 matching lines @@
   V(Exit, 1)                                                                   \
   V(File_Open, 2)                                                              \
   V(File_Exists, 1)                                                            \
   V(File_Close, 1)                                                             \
   V(File_ReadByte, 1)                                                          \
   V(File_WriteByte, 2)                                                         \
   V(File_WriteString, 2)                                                       \
   V(File_ReadList, 4)                                                          \
   V(File_WriteList, 4)                                                         \
   V(File_Position, 1)                                                          \
+  V(File_SetPosition, 2)                                                       \
   V(File_Length, 1)                                                            \
   V(File_Flush, 1)                                                             \
   V(File_Create, 1)                                                            \
   V(File_FullPath, 1)                                                          \
   V(Logger_PrintString, 1)                                                     \
   V(Platform_NumberOfProcessors, 0)                                            \
   V(Platform_OperatingSystem, 0)                                               \
   V(Platform_PathSeparator, 0)                                                 \
   V(Process_Start, 8)                                                          \
   V(Process_Kill, 2)                                                           \
@@ skipping 92 matching lines @@
 }
 
 
 void Builtin_SetNativeResolver() {
   Dart_Handle url = Dart_NewString(DartUtils::kBuiltinLibURL);
   Dart_Handle builtin_lib = Dart_LookupLibrary(url);
   DART_CHECK_VALID(builtin_lib);
   Dart_Handle result = Dart_SetNativeResolver(builtin_lib, native_lookup);
   DART_CHECK_VALID(result);
 }