Add test for JS_CONST

tests/compiler/dart2js_native/js_const_test.dart

+// Copyright (c) 2014, 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 'dart:_foreign_helper' show JS, JS_CONST;
+
+test1() {
+  var re = const JS_CONST(r'/-([\da-z])/ig');
+  var fToUpper = const JS_CONST(
+      r'function(_, letter){return letter.toUpperCase()}');
+  var s1 = '-hello-world';
+  var s2 = JS('String', r'#.replace(#, #)', s1, re, fToUpper);
+  Expect.equals('HelloWorld', s2);
+
+  s1 = 'hello-world';
+  s2 = JS('String', r'#.replace(#, #)', s1, re, fToUpper);
+  Expect.equals('helloWorld', s2);
+}
+
+main() {
+  test1();
+}