Turn a Dart String + String into a JavaScript String + String.

tests/compiler/dart2js/string_interpolation_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.
 
 import "package:expect/expect.dart";
 import "package:async_helper/async_helper.dart";
 import 'compiler_helper.dart';
 
 main() {
   asyncTest(() => compileAll(
       r'''main() { return "${2}${true}${'a'}${3.14}"; }''').then((code) {
     Expect.isTrue(code.contains(r'2truea3.14'));
   }));
 
   asyncTest(() => compileAll(
       r'''main() { return "foo ${new Object()}"; }''').then((code) {
-    Expect.isFalse(code.contains(r'concat'));
+    Expect.isFalse(code.contains(r'$add'));
   }));
 }