Add tests for constants from deferred loaded libraries being readable.

tests/language/deferred_load_constants_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.
+// Dart version of two-argument Ackermann-Peter function.
+
+library deferred_load_constants;
+
+import "package:expect/expect.dart";
+import "package:async_helper/async_helper.dart";
+import "deferred_load_constants.dart" deferred as foo;
+import "deferred_load_constants.dart";
+
+main() {
+  asyncStart();
+  Expect.throws(() => foo.c);
+  Expect.throws(() => foo.C);
+  Expect.throws(() => foo.funtype);
+  Expect.throws(() => foo.toplevel);
+  foo.loadLibrary().whenComplete(() {
+    // Reading constant declarations through deferred prefix works.
+    Expect.identical(c, foo.c);
+    Expect.identical(C, foo.C);
+    Expect.identical(funtype, foo.funtype);
+    Expect.identical(toplevel, foo.toplevel);
+    Expect.identical(C.staticfun, foo.C.staticfun);
+    // Access through deferred prefix is not a constant expression.
+    Expect.throws(() => const [foo.c]);            /// 01: compile-time error
+    Expect.throws(() => const [foo.C]);            /// 02: compile-time error
+    Expect.throws(() => const [foo.funtype]);      /// 03: compile-time error
+    Expect.throws(() => const [foo.toplevel]);     /// 04: compile-time error
+    Expect.throws(() => const [foo.C.staticfun]);  /// 05: compile-time error
+
+    asyncEnd();
+  });
+}