Move kernel baseline tests to front_end.

pkg/front_end/testcases/external.dart

 // Copyright (c) 2016, 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 'dart:isolate';
 
 var subscription;
 
 void onData(x) {
   print(x);
   subscription.cancel();
 }
 
 main() {
   var string = new String.fromCharCode(65); // External string factory.
   var port = new ReceivePort(); // External factory.
   subscription = port.listen(onData); // Dynamic call on external instance.
   port.sendPort.send(string);
 }