[VM/nnbd] Force the DeclarationType of a class to have legacy nullability.

This is a workaround to avoid caching a non-legacy type (possibly provided
by the CFE) as the canonical type of a non-generic class and later returned as
the runtime type of an instance of this class.

Hopefully fixes https://buganizer.corp.google.com/issues/144304690

Change-Id: Iaea364b9b743b05bde600f0aa784d5a8105728d7
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/124920
Reviewed-by: Alexander Markov <alexmarkov@google.com>
Commit-Queue: Régis Crelier <regis@google.com>

Author: crelier

runtime/vm/object.cc

@@ -4351,11 +4351,16 @@ RawType* Class::DeclarationType(Nullability nullability) const {
   if (!type.IsNull()) {
     return type.ToNullability(nullability, Heap::kOld);
   }
+  // TODO(regis): We should pass nullabiity to Type::New to avoid having to
+  // clone the type to the desired nullability. This however causes issues with
+  // the runtimeType intrinsic grabbing DeclarationType without checking its
+  // nullability. Indeed, when the CFE provides a non-nullable version of the
+  // type first, this non-nullable version gets cached as the declaration type.
   type = Type::New(*this, TypeArguments::Handle(type_parameters()), token_pos(),
-                   nullability);
+                   Nullability::kLegacy);
   type ^= ClassFinalizer::FinalizeType(*this, type);
   set_declaration_type(type);
-  return type.raw();
+  return type.ToNullability(nullability, Heap::kOld);
 }
 
 void Class::set_allocation_stub(const Code& value) const {