Migrated test block 20 to Dart 2.0.

tests/corelib_2/regexp/lastindex_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 24 matching lines @@
   for (var match in re.allMatches("  abcdefg"))
     accumulate += match.group(0) + "; ";
   assertEquals(accumulate, "abcdefg; ");
 
   re = new RegExp(r"\d");
   accumulate = "";
   for (var match in re.allMatches("123456789"))
     accumulate += match.group(0) + "; ";
   assertEquals(accumulate, "1; 2; 3; 4; 5; 6; 7; 8; 9; ");
 }