Implement setPosition on File to seek to a byte position.

runtime/bin/file_macos.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 <errno.h>
 #include <fcntl.h>
 #include <sys/stat.h>
 #include <unistd.h>
 #include <libgen.h>
 #include <limits.h>
@@ skipping 55 matching lines @@
   return write(handle_->fd(), buffer, num_bytes);
 }
 
 
 off_t File::Position() {
   ASSERT(handle_->fd() >= 0);
   return lseek(handle_->fd(), 0, SEEK_CUR);
 }
 
 
+off_t File::SetPosition(int64_t position) {
+  ASSERT(handle_->fd() >= 0);
+  return lseek(handle_->fd(), position, SEEK_SET);
+}
+
+
 void File::Flush() {
   ASSERT(handle_->fd() >= 0);
   fsync(handle_->fd());
 }
 
 
 off_t File::Length() {
   ASSERT(handle_->fd() >= 0);
   off_t position = lseek(handle_->fd(), 0, SEEK_CUR);
   if (position < 0) {
@@ skipping 60 matching lines @@
 
 
 const char* File::PathSeparator() {
   return "/";
 }
 
 
 const char* File::StringEscapedPathSeparator() {
   return "/";
 }