Make RegExp constructor non-const.

tests/compiler/dart2js/gvn_dynamic_field_get_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 that dart2js gvns dynamic getters that don't have side
 // effects.
 
 import 'compiler_helper.dart';
 import 'parser_helper.dart';
 import 'dart:uri';
 
 const String TEST = r"""
 class A {
   var foo;
   bar(a) {
     return a.foo + a.foo;
   }
 }
 
 main() {
   new A().bar(new Object());
 }
 """;
 
 main() {
   Uri uri = new Uri.fromComponents(scheme: 'source');
   var compiler = compilerFor(TEST, uri);
   compiler.runCompiler(uri);
   String generated = compiler.assembledCode;
-  RegExp regexp = const RegExp(r"get\$foo");
+  RegExp regexp = new RegExp(r"get\$foo");
   Iterator matches = regexp.allMatches(generated).iterator();
   checkNumberOfMatches(matches, 1);
   var cls = findElement(compiler, 'A');
   Expect.isNotNull(cls);
   var element = cls.lookupLocalMember(buildSourceString('foo'));
   Expect.isNotNull(element);
   Expect.isFalse(compiler.world.userDefinedGetters.contains(element));
 }