Make RegExp constructor non-const.

tests/compiler/dart2js/redundant_phi_eliminator_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.
 
 import 'compiler_helper.dart';
 
 const String TEST_ONE = r"""
 void foo(bar) {
   var toBeRemoved = 1;
   if (bar) {
@@ skipping 10 matching lines @@
   var toBeRemoved = temp;
   for (var i = 0; i == 0; i = i + 1) {
     toBeRemoved = temp;
   }
   print(toBeRemoved);
 }
 """;
 
 main() {
   String generated = compile(TEST_ONE, entry: 'foo');
-  RegExp regexp = const RegExp("toBeRemoved");
+  RegExp regexp = new RegExp("toBeRemoved");
   Expect.isTrue(!regexp.hasMatch(generated));
 
   generated = compile(TEST_TWO, entry: 'foo');
-  regexp = const RegExp("toBeRemoved");
+  regexp = new RegExp("toBeRemoved");
   Expect.isTrue(!regexp.hasMatch(generated));
-  regexp = const RegExp("temp");
+  regexp = new RegExp("temp");
   Expect.isTrue(!regexp.hasMatch(generated));
 }