Add tests for constants from deferred loaded libraries being readable.

tests/language/deferred_load_constants.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.
+// Dart version of two-argument Ackermann-Peter function.
+
+library deferred_load_constants;
+
+// Constant declaration.
+const c = const C();
+// Class declaration (`C` is a constant expression).
+class C {
+  const C();
+  static int staticfun(int x) => x;
+}
+// Function type definition.
+typedef int funtype(int x);
+// Top-level function.
+int toplevel(int x) => x;