Make RegExp's constructor non-const.

tests/compiler/dart2js/class_order_test.dart

 // Copyright (c) 2011, 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 that parameters keep their names in the output.
 
 import 'compiler_helper.dart';
 
 const String TEST_ONE = r"""
 class A { foo() => 499; }
 class B { bar() => 499; }
@@ skipping 14 matching lines @@
 void main() {
   new C().gee();
   new B().bar();
   new A().foo();
 }
 """;
 
 main() {
   // Make sure that class A, B and C are emitted in that order. For simplicity
   // we just verify that their members are in the correct order.
-  RegExp regexp = const RegExp(r"foo\$0?:(.|\n)*bar\$0:(.|\n)*gee\$0:");
+  RegExp regexp = new RegExp(r"foo\$0?:(.|\n)*bar\$0:(.|\n)*gee\$0:");
 
   String generated = compileAll(TEST_ONE);
   print(generated);
   Expect.isTrue(regexp.hasMatch(generated));
 
   generated = compileAll(TEST_TWO);
   Expect.isTrue(regexp.hasMatch(generated));
 }