Remove corelib/corelib_strong and migrate last two remaining tests.

tests/corelib_2/stacktrace_current_test.dart

 // Copyright (c) 2015, 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:convert" show LineSplitter;
 
 import "package:expect/expect.dart";
 
 void main() {
   var st0;
@@ skipping 16 matching lines @@
 }
 
 String findMain(StackTrace stack) {
   var string = "$stack";
   var lines = LineSplitter.split(string).toList();
   while (lines.isNotEmpty && !lines.first.contains("main")) {
     lines.removeAt(0);
   }
   return lines.join("\n");
 }