Fix deprecated StringBuffer usages in Intl

pkg/intl/lib/src/date_format_field.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 intl;
 
 /**
  * This is a private class internal to DateFormat which is used for formatting
  * particular fields in a template. e.g. if the format is hh:mm:ss then the
  * fields would be "hh", ":", "mm", ":", and "ss". Each type of field knows
@@ skipping 380 matching lines @@
 
  /**
   * Return a string representation of the object padded to the left with
   * zeros. Primarily useful for numbers.
   */
   String padTo(int width, Object toBePrinted) {
     var basicString = toBePrinted.toString();
     if (basicString.length >= width) return basicString;
     var buffer = new StringBuffer();
     for (var i = 0; i < width - basicString.length; i++) {
-      buffer.add('0');
+      buffer.write('0');
      }
-    buffer.add(basicString);
+    buffer.write(basicString);
     return buffer.toString();
   }
 }