Fix warnings spotted by dart.

dart/sdk/lib/_internal/compiler/implementation/util/uri_extras.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.
 
 library uri_extras;
 
 import 'dart:math';
 import 'dart:uri';
 
 String relativize(Uri base, Uri uri, bool isWindows) {
@@ skipping 35 matching lines @@
       common++;
     }
     if (common == 1 || (isWindows && common == 2)) {
       // The first part will always be an empty string because the
       // paths are absolute. On Windows, we must also consider drive
       // letters or hostnames.
       return uri.path;
     }
     StringBuffer sb = new StringBuffer();
     for (int i = common + 1; i < baseParts.length; i++) {
-      sb.add('../');
+      sb.write('../');
     }
     for (int i = common; i < uriParts.length - 1; i++) {
-      sb.add('${uriParts[i]}/');
+      sb.write('${uriParts[i]}/');
     }
-    sb.add('${uriParts.last}');
+    sb.write('${uriParts.last}');
     return sb.toString();
   }
   return uri.toString();
 }