Fix deprecation warnings in dart:io. Now completely warning free.

sdk/lib/io/file.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.
 
 part of dart.io;
 
 /**
  * FileMode describes the modes in which a file can be opened.
  */
 class FileMode {
@@ skipping 437 matching lines @@
    */
   String get path;
 }
 
 
 class FileIOException implements Exception {
   const FileIOException([String this.message = "",
                          OSError this.osError = null]);
   String toString() {
     StringBuffer sb = new StringBuffer();
-    sb.add("FileIOException");
+    sb.write("FileIOException");
     if (!message.isEmpty) {
-      sb.add(": $message");
+      sb.write(": $message");
       if (osError != null) {
-        sb.add(" ($osError)");
+        sb.write(" ($osError)");
       }
     } else if (osError != null) {
-      sb.add(": osError");
+      sb.write(": osError");
     }
     return sb.toString();
   }
   final String message;
   final OSError osError;
 }