dart2js: Support new const operations on strings.

tests/language/compile_time_constant_c_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.
 
 const m0 = const {
   499: 400 + 99
 };
 const m1 = const {
-  "foo" + "bar": 42            /// 01: compile-time error
+  "foo" + "bar": 42            /// 01: ok
 };
 const m2 = const {
   "foo" * 4: 42                /// 02: compile-time error
 };
 const m3 = const {
   "foo".codeUnitAt(0): 42      /// 03: compile-time error
 };
 
 use(x) => x;
 
 main() {
   use(m0);
   use(m1);
   use(m2);
   use(m3);
 }