Remove deprecated StringBuffer.add, addAll and addCharCode.

tests/compiler/dart2js/deprecated_features_test.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 that deprecated language features are diagnosed correctly.
 
 import 'dart:async';
 import 'dart:uri';
 
 import '../../../sdk/lib/_internal/compiler/compiler.dart';
 import '../../utils/dummy_compiler_test.dart' as dummy;
 
 main() {
   StringBuffer messages = new StringBuffer();
   void handler(Uri uri, int begin, int end, String message, Diagnostic kind) {
     if (kind == Diagnostic.VERBOSE_INFO) return;
     if (identical(kind.name, 'source map')) return;
     if (uri == null) {
-      messages.add('$kind: $message\n');
+      messages.write('$kind: $message\n');
     } else {
       Expect.equals('main:${uri.path}', '$uri');
       String source = TEST_SOURCE[uri.path];
       Expect.isNotNull(source);
-      messages.add('$begin<${source.substring(begin, end)}>:${uri.path}:'
+      messages.write('$begin<${source.substring(begin, end)}>:${uri.path}:'
                    '$kind: $message\n');
     }
   }
 
   Future<String> provider(Uri uri) {
     if (uri.scheme != "main") return dummy.provider(uri);
     String source = TEST_SOURCE[uri.path];
     Expect.isNotNull(source);
     return (new Completer<String>()..complete(source)).future;
   }
@@ skipping 65 matching lines @@
 main() {
   var a = Foo.bar();
   var b = new Foo.bar();
   new Fisk();
   new Fisk.hest();
 }
 """,
     // TODO(ahe): Why isn't this 'part.dart'? Why the leading slash?
     '/part.dart': '',
   };