Test file write of sub-list.

tests/standalone/io/file_test.dart

 // Copyright (c) 2013, 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.
 //
 // Dart test program for testing file I/O.
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:collection';
 import 'dart:io';
@@ skipping 331 matching lines @@
     openedFile.writeFromSync(buffer, 0, buffer.length);
     openedFile.closeSync();
     // Reopen the file in write mode to ensure that we overwrite the content.
     openedFile = (new File(filename)).openSync(mode: WRITE);
     openedFile.writeFromSync(buffer, 0, buffer.length);
     Expect.equals(content.length, openedFile.lengthSync());
     openedFile.closeSync();
     // Open the file in append mode and ensure that we do not overwrite
     // the existing content.
     openedFile = (new File(filename)).openSync(mode: APPEND);
-    openedFile.writeFromSync(buffer, 0, buffer.length);
-    Expect.equals(content.length * 2, openedFile.lengthSync());
+    openedFile.writeFromSync(buffer, 2, buffer.length - 2);
+    Expect.equals(content.length + content.length - 4,
+                  openedFile.lengthSync());
+    Expect.equals(content + content.substring(2, content.length - 2),
+                  file.readAsStringSync());
     openedFile.closeSync();
     file.deleteSync();
   }
 
   static void testOutputStreamWriteAppend() {
     asyncTestStarted();
     String content = "foobar";
     String filename = tempDirectory.path + "/outstream_write_append";
     File file = new File(filename);
     file.createSync();
@@ skipping 962 matching lines @@
       testLastModified();
       testDoubleAsyncOperation();
       asyncEnd();
     });
   }
 }
 
 main() {
   FileTest.testMain();
 }