[cfe] Add test for issue for opt-in import of opt-out typedef

Closes #42181

Change-Id: I1557a5843cba6001580d8ff19cd1864e7bc37c48
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/155400
Reviewed-by: Dmitry Stefantsov <[email protected]>

Author: johnniwinther

pkg/front_end/testcases/nnbd_mixed/issue42181.dart

@@ -0,0 +1,14 @@
+// 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 'issue42181_lib.dart';
+
+int? f1(int x) => x;
+
+void test() {
+  F f = null; // Static error
+  f = f1; // No error
+}
+
+main() {}

pkg/front_end/testcases/nnbd_mixed/issue42181.dart.textual_outline.expect

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+import 'issue42181_lib.dart';
+
+int? f1(int x) => x;
+void test() {}
+main() {}

pkg/front_end/testcases/nnbd_mixed/issue42181.dart.textual_outline_modelled.expect

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+import 'issue42181_lib.dart';
+
+int? f1(int x) => x;
+main() {}
+void test() {}

pkg/front_end/testcases/nnbd_mixed/issue42181.dart.weak.expect

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+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/nnbd_mixed/issue42181.dart:10:9: Error: A value of type 'Null' can't be assigned to a variable of type 'int Function(int)'.
+//   F f = null; // Static error
+//         ^
+//
+import self as self;
+import "dart:core" as core;
+
+import "org-dartlang-testcase:///issue42181_lib.dart";
+
+static method f1(core::int x) → core::int?
+  return x;
+static method test() → void {
+  (core::int*) → core::int* f = let final<BottomType> #t1 = invalid-expression "pkg/front_end/testcases/nnbd_mixed/issue42181.dart:10:9: Error: A value of type 'Null' can't be assigned to a variable of type 'int Function(int)'.
+  F f = null; // Static error
+        ^" in null as{TypeError,ForNonNullableByDefault} (core::int*) → core::int*;
+  f = #C1;
+}
+static method main() → dynamic {}
+
+library;
+import self as self2;
+import "dart:core" as core;
+
+typedef F = (core::int*) →* core::int*;
+
+constants  {
+  #C1 = tearoff self::f1
+}

pkg/front_end/testcases/nnbd_mixed/issue42181.dart.weak.transformed.expect

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+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/nnbd_mixed/issue42181.dart:10:9: Error: A value of type 'Null' can't be assigned to a variable of type 'int Function(int)'.
+//   F f = null; // Static error
+//         ^
+//
+import self as self;
+import "dart:core" as core;
+
+import "org-dartlang-testcase:///issue42181_lib.dart";
+
+static method f1(core::int x) → core::int?
+  return x;
+static method test() → void {
+  (core::int*) → core::int* f = let final<BottomType> #t1 = invalid-expression "pkg/front_end/testcases/nnbd_mixed/issue42181.dart:10:9: Error: A value of type 'Null' can't be assigned to a variable of type 'int Function(int)'.
+  F f = null; // Static error
+        ^" in null;
+  f = #C1;
+}
+static method main() → dynamic {}
+
+library;
+import self as self2;
+import "dart:core" as core;
+
+typedef F = (core::int*) →* core::int*;
+
+constants  {
+  #C1 = tearoff self::f1
+}

pkg/front_end/testcases/nnbd_mixed/issue42181_lib.dart

@@ -0,0 +1,7 @@
+// 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.
+
+// @dart=2.8
+
+typedef F = int Function(int);