[vm/aot/nnbd] Fix class range 'is' tests for nullable types

Issue: https://github.com/flutter/flutter/issues/63819
Change-Id: I7d3fe2a3f6c40b4a5bae9b1ba3ab1dd36fab8f5c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/158800
Reviewed-by: Vyacheslav Egorov <vegorov@google.com>
Commit-Queue: Alexander Markov <alexmarkov@google.com>

runtime/tests/vm/dart/regress_flutter63819_test.dart

@@ -0,0 +1,31 @@
+// 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.
+
+// Verify that 'is' test of a nullable type accepts null.
+// Regression test for https://github.com/flutter/flutter/issues/63819.
+
+import 'package:expect/expect.dart';
+
+abstract class A {}
+
+class B extends A {}
+
+class C extends A {}
+
+@pragma('vm:never-inline')
+bool foo(A? x) {
+  if (x is C?) {
+    print('$x is C?');
+    return true;
+  }
+  print('$x is not C?');
+  return false;
+}
+
+void main() {
+  B();
+  C();
+  Expect.isFalse(foo(B()));
+  Expect.isTrue(foo(null));
+}

runtime/vm/compiler/backend/il.cc

@@ -453,6 +453,12 @@ bool HierarchyInfo::InstanceOfHasClassRange(const AbstractType& type,
                                             intptr_t* lower_limit,
                                             intptr_t* upper_limit) {
   ASSERT(CompilerState::Current().is_aot());
+  if (type.IsNullable()) {
+    // 'is' test for nullable types should accept null cid in addition to the
+    // class range. In most cases it is not possible to extend class range to
+    // include kNullCid.
+    return false;
+  }
   if (CanUseSubtypeRangeCheckFor(type)) {
     const Class& type_class =
         Class::Handle(thread()->zone(), type.type_class());