Change the default for the len argument to writeFrom on OutputStream

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 55 matching lines @@
   File _file;
 }
 
 
 class _FileOutputStream implements FileOutputStream {
   _FileOutputStream(File file) {
     _file = new File(file.name);
     _file.openSync(true);
   }
 
-  bool write(List<int> buffer) {
+  bool write(List<int> buffer, [bool copyBuffer = false]) {
     return _write(buffer, 0, buffer.length);
   }
 
   bool writeFrom(List<int> buffer, [int offset, int len]) {
-    return _write(buffer, offset, (len == null) ? buffer.length : len);
+    return _write(
+        buffer, offset, (len == null) ? buffer.length - offset : len);
   }
 
   void end() {
     _file.closeSync();
   }
 
   void close() {
     _file.closeSync();
   }
 
@@ skipping 758 matching lines @@
   var _closeHandler;
   var _readByteHandler;
   var _readListHandler;
   var _noPendingWriteHandler;
   var _positionHandler;
   var _lengthHandler;
   var _flushHandler;
   var _fullPathHandler;
   var _errorHandler;
 }