Add isNotNull and isNull checks.

Change-Id: Ib8fb64e15dcc2458667d48cd6c86c83f6b45d25c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/223840
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Reviewed-by: Samuel Rawlins <srawlins@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>

pkg/analyzer_utilities/lib/check/check.dart

@@ -10,6 +10,7 @@ export 'package:analyzer_utilities/check/check_target.dart';
 export 'package:analyzer_utilities/check/equality.dart';
 export 'package:analyzer_utilities/check/int.dart';
 export 'package:analyzer_utilities/check/iterable.dart';
+export 'package:analyzer_utilities/check/nullability.dart';
 export 'package:analyzer_utilities/check/string.dart';
 export 'package:analyzer_utilities/check/type.dart';
 

pkg/analyzer_utilities/lib/check/nullability.dart

@@ -0,0 +1,21 @@
+// Copyright (c) 2021, 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:analyzer_utilities/check/check.dart';
+
+extension NullabilityExtension<T> on CheckTarget<T?> {
+  void get isNull {
+    if (value != null) {
+      fail('is not null');
+    }
+  }
+
+  CheckTarget<T> get isNotNull {
+    final value = this.value;
+    if (value == null) {
+      fail('is null');
+    }
+    return nest(value, (value) => 'is not null');
+  }
+}

pkg/analyzer_utilities/test/check/check_test.dart

@@ -97,6 +97,18 @@ void main() {
         _fails(() => check(<int>{}).isNotEmpty);
       });
     });
+    group('nullability', () {
+      const int? notNullable = 0;
+      const int? nullable = null;
+      test('isNotNull', () {
+        check(notNullable).isNotNull;
+        _fails(() => check(nullable).isNotNull.isZero);
+      });
+      test('isNull', () {
+        check(nullable).isNull;
+        _fails(() => check(notNullable).isNull);
+      });
+    });
     group('String', () {
       test('contains', () {
         check('abc').contains('a');