[tests] Create null is T tests in legacy and null safe libraries

Change-Id: I2054b6da8b66e7c6de81d9fb18fdc1c264db376c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/132420
Commit-Queue: Nicholas Shahan <nshahan@google.com>
Reviewed-by: Leaf Petersen <leafp@google.com>
Reviewed-by: Erik Ernst <eernst@google.com>

tests/language/nnbd/is_type_test/legacy_library.dart

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+// 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.
+
+// Opt out of Null Safety:
+// @dart = 2.6
+
+import 'null_safe_library.dart';
+
+/// Performs the type test [value] is [T] in a legacy library.
+///
+/// NOTE: The [T] here is in a legacy library and will become `T*` which might
+/// be normalized away depending on the value of `T`.
+bool legacyIs<T>(Object value) => value is T;
+
+/// Performs the type test [value] is [T] in a null safe library.
+///
+/// NOTE: The [T] here is in a legacy library and will become `T*` which might
+/// be normalized away depending on the value of `T`.
+bool nullSafeIsLegacy<T>(Object value) => nullSafeIs<T>(value);

tests/language/nnbd/is_type_test/null_is_type_in_legacy_lib_test.dart

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+// 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.
+
+// Opt out of Null Safety:
+// @dart = 2.6
+
+import 'package:expect/expect.dart';
+import 'null_safe_library.dart';
+
+// Type tests (is checks) located in a legacy library.
+main() {
+  // `null is Never*`
+  Expect.isTrue(null is Never);
+  // `null is Never?`
+  Expect.isTrue(legacyIsNullable<Never>(null));
+  Expect.isTrue(null is Null);
+  // `null is int*`
+  Expect.isFalse(null is int);
+  // `null is int?`
+  Expect.isTrue(legacyIsNullable<int>(null));
+  // `null is Object*`
+  Expect.isTrue(null is Object);
+  // `null is Object?`
+  Expect.isTrue(legacyIsNullable<Object>(null));
+  Expect.isTrue(null is dynamic);
+}

tests/language/nnbd/is_type_test/null_is_type_in_null_safe_lib_test.dart

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+// 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';
+import 'legacy_library.dart';
+
+// Type tests (is checks) located in a null safe library.
+main() {
+  Expect.isFalse(null is Never);
+  // `null is Never*`
+  Expect.isTrue(nullSafeIsLegacy<Never>(null));
+  Expect.isTrue(null is Never?);
+  Expect.isTrue(null is Null);
+  Expect.isFalse(null is int);
+  // `null is int*`
+  Expect.isFalse(nullSafeIsLegacy<int>(null));
+  Expect.isTrue(null is int?);
+  Expect.isFalse(null is Object);
+  // `null is Object*`
+  Expect.isTrue(nullSafeIsLegacy<Object>(null));
+  Expect.isTrue(null is Object?);
+  Expect.isTrue(null is dynamic);
+}

tests/language/nnbd/is_type_test/null_safe_library.dart

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+// 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 'legacy_library.dart';
+
+/// Performs the type test [value] is [T] in a null safe library.
+bool nullSafeIs<T>(Object? value) => value is T;
+
+/// Performs the type test [value] is `T?` in a legacy library.
+bool legacyIsNullable<T>(Object? value) => legacyIs<T?>(value);