Migrated test block 20 to Dart 2.0.

tests/corelib_2/regexp/loop-capture_test.dart

 // Copyright (c) 2014, the Dart project authors. All rights reserved.
 // Copyright 2009 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
@@ skipping 16 matching lines @@
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 import 'v8_regexp_utils.dart';
 import 'package:expect/expect.dart';
 
 void main() {
   shouldBe(new RegExp(r"(?:(a)|(b)|(c))+").firstMatch("abc"),
       ["abc", null, null, "c"]);
   shouldBe(new RegExp(r"(?:(a)|b)*").firstMatch("ab"), ["ab", null]);
 }