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

tests/utils/dummy_compiler_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.
 // VMOptions=
 // VMOptions=--print-object-histogram
 
 // Smoke test of the dart2js compiler API.
 library dummy_compiler;
 
 import 'dart:async';
@@ skipping 49 matching lines @@
 class JSArray implements JSIndexable {
   var removeLast;
   var add;
 }
 class JSMutableArray extends JSArray {}
 class JSFixedArray extends JSMutableArray {}
 class JSExtendableArray extends JSMutableArray {}
 class JSString implements JSIndexable {
   var split;
   var concat;
+  operator+(other) {}
   var toString;
 }
 class JSFunction {}
 class JSInt {}
 class JSDouble {}
 class JSNumber {}
 class JSNull {}
 class JSBool {}
 getInterceptor(o){}
 getDispatchProperty(o) {}
@@ skipping 30 matching lines @@
               new Uri(scheme: 'package', path: '/'),
               provider, handler);
   result.then((String code) {
     if (code == null) {
       throw 'Compilation failed';
     }
   }, onError: (e) {
       throw 'Compilation failed';
   }).whenComplete(() => asyncEnd());
 }