Remove corelib/corelib_strong and migrate last two remaining tests.

tests/corelib_strong/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;
-  var st1;
-  // Primitive way to get stack trace,.
-  try {
-    throw 0;
-  } catch (_, s) {
-    st0 = s;
-  }
-  st1 = StackTrace.current;
-
-  var st0s = findMain(st0);
-  var st1s = findMain(st1);
-  // Stack traces are not equal (contains at least a different line number,
-  // and possible different frame numbers).
-  // They are *similar*, so check that they agree on everything but numbers.
-  var digits = new RegExp(r"\d+");
-  Expect.equals(st0s.replaceAll(digits, "0"), st1s.replaceAll(digits, "0"));
-}
-
-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");
-}