Remove deprecated StringBuffer.add, addAll and addCharCode.

tests/language/call_through_getter_test.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.
 
 // Tests that we can call functions through getters.
 
 const TOP_LEVEL_CONST = 1;
 const TOP_LEVEL_CONST_REF = TOP_LEVEL_CONST;
 const TOP_LEVEL_NULL = null;
 
@@ skipping 137 matching lines @@
 
   get g0 { _mark('g'); return () { return _mark('f'); }; }
   get g1 { _mark('g'); return (x) { return _mark('f'); }; }
   get g2 { _mark('g'); return (x, y) { return _mark('f'); }; }
   get g3 { _mark('g'); return (x, y, z) { return _mark('f'); }; }
 
   get x { _mark('x'); return 0; }
   get y { _mark('y'); return 1; }
   get z { _mark('z'); return 2; }
 
-  _mark(m) { _order.add(m); return _order.toString(); }
+  _mark(m) { _order.write(m); return _order.toString(); }
   StringBuffer _order;
 
 }
 
 main() {
   CallThroughGetterTest.testMain();
 }