Migrated test block 29 to Dart 2.0

tests/corelib_2/string_to_lower_case_test.dart

 // Copyright (c) 2014, 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 "package:expect/expect.dart";
 
 void testOneByteSting() {
   // Compare one-byte-string toLowerCase with a two-byte-string toLowerCase.
   var oneByteString =
       new String.fromCharCodes(new List.generate(256, (i) => i)).toLowerCase();
   var twoByteString =
       new String.fromCharCodes(new List.generate(512, (i) => i)).toLowerCase();
   Expect.isTrue(twoByteString.codeUnits.any((u) => u >= 256));
   Expect.equals(oneByteString, twoByteString.substring(0, 256));
 }
 
 void main() {
   testOneByteSting();
 }