Revert "Remove NullPointerException."

tests/language/string_interpolate_null_test.dart

-// Copyright (c) 2011, 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.
-// Dart test program testing NPE within string interpolation.
-
-class A {
-  A(String this.name) {}
-  String name;
-}
-
-main() {
-  A a = new A("Kermit");
-  var s = "Hello Mr. ${a.name}";
-  Expect.stringEquals("Hello Mr. Kermit", s);
-  a = null;
-  try {
-    s = "Hello Mr. ${a.name}";
-  } on NoSuchMethodError catch (e) {
-    return;
-  }
-  Expect.fail("NoSuchMethodError not thrown");
-}