Implement setPosition on File to seek to a byte position.

runtime/bin/file.h

 // 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.
 
 #ifndef BIN_FILE_H_
 #define BIN_FILE_H_
 
 #if defined(_WIN32)
 typedef signed __int64 int64_t;
 typedef unsigned __int8 uint8_t;
@@ skipping 28 matching lines @@
     return WriteFully(&byte, 1);
   }
 
   // Get the length of the file. Returns a negative value if the length cannot
   // be determined (e.g. not seekable device).
   off_t Length();
 
   // Get the current position in the file.
   // Returns a negative value if position cannot be determined.
   off_t Position();
+  off_t SetPosition(int64_t position);
 
   // Flush contents of file.
   void Flush();
 
   const char* name() const { return name_; }
 
   static File* Open(const char* name, bool writable);
   static bool Exists(const char* name);
   static bool Create(const char* name);
   static bool IsAbsolutePath(const char* pathname);
@@ skipping 11 matching lines @@
   const char* name_;
   // FileHandle is an OS specific class which stores data about the file.
   FileHandle* handle_;  // OS specific handle for the file.
 
   // DISALLOW_COPY_AND_ASSIGN(File).
   File(const File&);
   void operator=(const File&);
 };
 
 #endif  // BIN_FILE_H_