Implement setPosition on File to seek to a byte position.

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 262 matching lines @@
   _PositionOperation(int this._id);
 
   void execute(ReceivePort port) {
     _replyPort.send(_File._position(_id), port.toSendPort());
   }
 
   int _id;
 }
 
 
+class _SetPositionOperation extends _FileOperation {
+  _SetPositionOperation(int this._id, int this._position);
+
+  void execute(ReceivePort port) {
+    _replyPort.send(_File._setPosition(_id, _position), port.toSendPort());
+  }
+
+  int _id;
+  int _position;
+}
+
+
 class _LengthOperation extends _FileOperation {
   _LengthOperation(int this._id);
 
   void execute(ReceivePort port) {
     _replyPort.send(_File._length(_id), port.toSendPort());
   }
 
   int _id;
 }
 
@@ skipping 112 matching lines @@
   static int _open(String name, bool writable) native "File_Open";
   static int _close(int id) native "File_Close";
   static int _readByte(int id) native "File_ReadByte";
   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 _setPosition(int id, int position) native "File_SetPosition";
   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 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;
@@ skipping 327 matching lines @@
       throw new FileIOException(
           "Mixed use of synchronous and asynchronous API");
     }
     int result = _position(_id);
     if (result == -1) {
       throw new FileIOException("position failed");
     }
     return result;
   }
 
+  void setPosition(int position) {
+    _asyncUsed = true;
+    var handler =
+        (_setPositionHandler != null) ? _setPositionHandler : () => null;
+    var handleSetPositionResult = (result, ignored) {
+      if (result == -1 && _errorHandler != null) {
+        _errorHandler("setPosition failed");
+        return;
+      }
+      handler(result);
+    };
+    var operation = new _SetPositionOperation(_id, position);
+    _scheduler.enqueue(operation, handleSetPositionResult);
+  }
+
+  void setPositionSync(int position) {
+    if (_asyncUsed) {
+      throw new FileIOException(
+          "Mixed use of synchronous and asynchronous API");
+    }
+    int result = _setPosition(_id, position);
+    if (result == -1) {
+      throw new FileIOException("setPosition failed");
+    }
+  }
+  
+
   void length() {
     _asyncUsed = true;
     var handler = (_lengthHandler != null) ? _lengthHandler : (pos) => null;
     var handleLengthResult = (result, ignored) {
       if (result == -1 && _errorHandler != null) {
         _errorHandler("length failed");
         return;
       }
       handler(result);
     };
@@ skipping 102 matching lines @@
   }
 
   void set noPendingWriteHandler(void handler()) {
     _noPendingWriteHandler = handler;
   }
 
   void set positionHandler(void handler(int pos)) {
     _positionHandler = handler;
   }
 
+  void set setPositionHandler(void handler()) {
+    _setPositionHandler = handler;
+  }
+
   void set lengthHandler(void handler(int length)) {
     _lengthHandler = handler;
   }
 
   void set flushHandler(void handler()) {
     _flushHandler = handler;
   }
 
   void set fullPathHandler(void handler(String)) {
     _fullPathHandler = handler;
@@ skipping 10 matching lines @@
   _FileOperationScheduler _scheduler;
 
   var _existsHandler;
   var _createHandler;
   var _openHandler;
   var _closeHandler;
   var _readByteHandler;
   var _readListHandler;
   var _noPendingWriteHandler;
   var _positionHandler;
+  var _setPositionHandler;
   var _lengthHandler;
   var _flushHandler;
   var _fullPathHandler;
   var _errorHandler;
 }