Migrated test block 20 to Dart 2.0.

tests/corelib_2/regexp/non-capturing-backtracking_test.dart

 // Copyright (c) 2014, the Dart project authors. All rights reserved.
 // Copyright 2013 the V8 project authors. All rights reserved.
 // Copyright (C) 2005, 2006, 2007, 2008, 2009 Apple Inc. All rights reserved.
 //
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions
 // are met:
 // 1.  Redistributions of source code must retain the above copyright
 //     notice, this list of conditions and the following disclaimer.
 // 2.  Redistributions in binary form must reproduce the above copyright
@@ skipping 14 matching lines @@
 import 'v8_regexp_utils.dart';
 import 'package:expect/expect.dart';
 
 void main() {
   description(
       "This page tests for proper backtracking with greedy quantifiers and non-capturing parentheses.");
 
   var re = new RegExp(r"(?:a*)a");
   shouldBe(re.firstMatch('a'), ['a']);
 }