Add nnbd_mode_test to package:expect.

This test confirms that the NNBD mode predicates are consistent with
equivalent dynamic tests.

Change-Id: Ifeb2b9a14144ce3bee10580c80fd4a1aeea43c32
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/148581
Reviewed-by: Stephen Adams <sra@google.com>
Commit-Queue: Mayank Patke <fishythefish@google.com>

pkg/expect/test/nnbd_mode_test.dart

@@ -0,0 +1,19 @@
+// Copyright (c) 2020, 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";
+
+final bool strong = () {
+  try {
+    int i = null as dynamic;
+    return false;
+  } catch (e) {
+    return true;
+  }
+}();
+
+void main() {
+  Expect.equals(strong, isStrongMode);
+  Expect.equals(!strong, isWeakMode);
+}