Migrate test block 47 and downstream dependencies to Dart 2.0.

tests/language_strong/checked_null_test.dart

-// Copyright (c) 2012, 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";
-
-class A {
-  Map a;
-  Comparator b;
-  // This code exhibited a bug in dart2js checked mode, where the type
-  // of [a] was inferred to be [Comparator] or null;
-  A()
-      : b = null,
-        a = null;
-}
-
-main() {
-  Expect.throws(bar); //# 01: continued
-}
-
-bar() {
-  // We would create a typed selector for the call to foo, where the
-  // receiver type is a typedef. Some code in the dart2js backend were
-  // not dealing correctly with typedefs and lead the compiler to
-  // crash.
-  new A().a.foo(); //# 01: static type warning
-}