Change file opening operation to set position at the end of the file when FileMode.APPEND is used.

runtime/bin/file_impl.dart

 // 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.
 
 class _FileInputStream extends _BaseDataInputStream implements InputStream {
   _FileInputStream(File file) {
     _file = file.openSync();
     _length = _file.lengthSync();
     _streamMarkedClosed = true;
     _checkScheduleCallbacks();
@@ skipping 31 matching lines @@
     _closed = true;
   }
 
   RandomAccessFile _file;
   int _length;
   bool _closed = false;
 }
 
 
 class _FileOutputStream implements OutputStream {
-  _FileOutputStream(File file) {
-    _file = file.openSync(FileMode.WRITE);
+  _FileOutputStream(File file, FileMode mode) {
+    _file = file.openSync(mode);
   }
 
   bool write(List<int> buffer, [bool copyBuffer = false]) {
     return _write(buffer, 0, buffer.length);
   }
 
   bool writeFrom(List<int> buffer, [int offset = 0, int len]) {
     return _write(
         buffer, offset, (len == null) ? buffer.length - offset : len);
   }
@@ skipping 576 matching lines @@
     }
     String result = _FileUtils.checkedFullPath(_name);
     if (result == null) {
       throw new FileIOException("fullPath failed");
     }
     return result;
   }
 
   InputStream openInputStream() => new _FileInputStream(this);
 
-  OutputStream openOutputStream() => new _FileOutputStream(this);
+  OutputStream openOutputStream([FileMode mode = FileMode.WRITE]) {
+    if (mode != FileMode.WRITE &&
+        mode != FileMode.APPEND) {
+      throw new FileIOException(
+          "Wrong FileMode. Use FileMode.WRITE or FileMode.APPEND");
+    }
+    return new _FileOutputStream(this, mode);
+  }
 
   String get name() => _name;
 
   void set existsHandler(void handler(bool exists)) {
     _existsHandler = handler;
   }
 
   void set createHandler(void handler()) {
     _createHandler = handler;
   }
@@ skipping 429 matching lines @@
   var _readByteHandler;
   var _readListHandler;
   var _noPendingWriteHandler;
   var _positionHandler;
   var _setPositionHandler;
   var _truncateHandler;
   var _lengthHandler;
   var _flushHandler;
   var _errorHandler;
 }