Test field promotion during top level inference.

Bug: https://github.com/dart-lang/language/issues/2020
Change-Id: I58d7f5691a4af614bcd481a109e340ca709fe4ec
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/270760
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>

tests/language/inference_update_2/top_level_type_inference_test.dart

@@ -0,0 +1,98 @@
+// Copyright (c) 2022, 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.
+
+// Tests that field promotion works during top level type inference.  In the
+// CFE, top level types are inferred earlier than method bodies, so this
+// verifies that the data structures necessary to support field promotion have
+// been initialized in time.
+
+// SharedOptions=--enable-experiment=inference-update-2
+
+class C {
+  final int? _promotable;
+  final int? _notPromotable; // due to D._notPromotable
+
+  C(int i)
+      : _promotable = i,
+        _notPromotable = i;
+
+  static final staticPromotable =
+      ((C c) => c._promotable != null ? c._promotable : 0)(new C(0));
+
+  static final staticNotPromotable =
+      ((C c) => c._notPromotable != null ? c._notPromotable : 0)(new C(0));
+
+  final instancePromotable =
+      ((C c) => c._promotable != null ? c._promotable : 0)(new C(0));
+
+  final instanceNotPromotable =
+      ((C c) => c._notPromotable != null ? c._notPromotable : 0)(new C(0));
+
+  late final instancePromotableViaThis = _promotable != null ? _promotable : 0;
+
+  late final instanceNotPromotableViaThis =
+      _notPromotable != null ? _notPromotable : 0;
+}
+
+class D {
+  int? _notPromotable;
+}
+
+final topLevelPromotable =
+    ((C c) => c._promotable != null ? c._promotable : 0)(new C(0));
+
+final topLevelNotPromotable =
+    ((C c) => c._notPromotable != null ? c._notPromotable : 0)(new C(0));
+
+void acceptsInt(int x) {}
+
+void testTopLevelPromotable() {
+  var x = topLevelPromotable;
+  // `x` has type `int` so this is ok
+  acceptsInt(x);
+}
+
+void testTopLevelNotPromotable() {
+  var x = topLevelNotPromotable;
+  // `x` has type `int?` so this is ok
+  x = null;
+}
+
+void testStaticPromotable() {
+  var x = C.staticPromotable;
+  // `x` has type `int` so this is ok
+  acceptsInt(x);
+}
+
+void testStaticNotPromotable() {
+  var x = C.staticNotPromotable;
+  // `x` has type `int?` so this is ok
+  x = null;
+}
+
+void testInstancePromotable(C c) {
+  var x = c.instancePromotable;
+  // `x` has type `int` so this is ok
+  acceptsInt(x);
+}
+
+void testInstanceNotPromotable(C c) {
+  var x = c.instanceNotPromotable;
+  // `x` has type `int?` so this is ok
+  x = null;
+}
+
+void testInstancePromotableViaThis(C c) {
+  var x = c.instancePromotableViaThis;
+  // `x` has type `int` so this is ok
+  acceptsInt(x);
+}
+
+void testInstanceNotPromotableViaThis(C c) {
+  var x = c.instanceNotPromotableViaThis;
+  // `x` has type `int?` so this is ok
+  x = null;
+}
+
+main() {}