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

tests/utils/recursive_import_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 of "recursive" imports using the dart2js compiler API.
 
 import "package:expect/expect.dart";
 import "package:async_helper/async_helper.dart";
 import 'dart:async';
 import '../../sdk/lib/_internal/compiler/compiler.dart';
@@ skipping 36 matching lines @@
   removeLast() => null;
   add(x) { }
 }
 class JSMutableArray extends JSArray {}
 class JSExtendableArray extends JSMutableArray {}
 class JSFixedArray extends JSMutableArray {}
 class JSString {
   split(x) => null;
   concat(x) => null;
   toString() => null;
+  operator+(other) => null;
 }
 class JSNull {
 }
 class JSBool {
 }
 """;
 
 const String RECURSIVE_MAIN = """
 library fisk;
 import 'recurse/fisk.dart';
@@ skipping 55 matching lines @@
     Expect.isNull(code);
     Expect.isTrue(10 < count);
     // Two warnings for each time RECURSIVE_MAIN is read, except the
     // first time.
     Expect.equals(2 * (count - 1), warningCount);
     Expect.equals(1, errorCount);
   }, onError: (e) {
       throw 'Compilation failed';
   }).whenComplete(() => asyncEnd());
 }