Remove deprecated StringBuffer.add, addAll and addCharCode.

tests/language/string_interpolation_and_buffer.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.
 
 // Test to ensure that StringBuffer and string interpolation behaves
 // the same and fail fast.
 
 class ToStringWrapper {
   final value;
 
@@ skipping 23 matching lines @@
       }
     }
     Expect.isTrue(result is String);
     return 'Success';
   }
 
   buffer(object) {
     var sb;
     if (checkedMode && object != null) {
       try {
-        sb = new StringBuffer().add(wrap(object));
+        sb = new StringBuffer()..write(wrap(object));
       } on TypeError {
         return 'Error';
       }
     } else {
       try {
-        sb = new StringBuffer().add(wrap(object));
+        sb = new StringBuffer()..write(wrap(object));
       } on ArgumentError {
         return 'Error';
       }
       Expect.isTrue(sb.toString() is String);
     }
     return 'Success';
   }
 
   initBuffer(object) {
     var sb;
@@ skipping 28 matching lines @@
   Expect.equals('Error', buffer([1]));
   Expect.equals('Error', buffer(new Object()));
 
   Expect.equals('Error', initBuffer(null));
   Expect.equals('Success', initBuffer(""));
   Expect.equals('Success', initBuffer("string"));
   Expect.equals('Error', initBuffer([]));
   Expect.equals('Error', initBuffer([1]));
   Expect.equals('Error', initBuffer(new Object()));
 }