Migrate test block 3 to Dart 2.0.

tests/corelib_2/bit_twiddling_test.dart

 // Copyright (c) 2013, 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.
 // Testing Bigints.
 
 library bit_twiddling_test;
 
 import "package:expect/expect.dart";
 
 bool haveBigints() {
@@ skipping 153 matching lines @@
   checkS(2, 3, 2);
   checkS(3, 3, 3);
   checkS(4, 3, -4);
 }
 
 main() {
   testBitLength();
   testToUnsigned();
   testToSigned();
 }