Add non-null types list to StrongModeOptions

pkg/analyzer/test/src/task/strong/non_null_checker_test.dart

 // Copyright (c) 2016, 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.
 
 // These tests are for an experimental feature that treats Dart primitive types
 // (int, bool, double, etc.) as non-nullable. This file is not evidence for an
 // intention to officially support non-nullable primitives in Dart (or general
 // NNBD, for that matter) so don't get too crazy about it.
 
 library analyzer.test.src.task.non_null_primitives.checker_test;
@@ skipping 24 matching lines @@
 }
 
 main() {
   var languages = new MyList();
   for (int i = 0; i < languages.length; ++i) {
     print(languages[i]);
   }
 }
 ''');
   }
+
+  void test_nullableTypes() {
+    // By default x can be set to null.
+    checkFile('''int x = null;''');
+  }
+
+  @failingTest
+  void test_nonnullableTypes() {
+    // If `int`s are non-nullable, then this code should throw an error.
+    addFile('''int x = /*error:INVALID_ASSIGNMENT*/null;''');
+    check(nonnullableTypes: <String>['dart:core,int']);
+  }
 }