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

This reverts commit 3771ddd.

Reason for revert: see https://buganizer.corp.google.com/issues/144304690
(This CL is dependent on https://dart-review.googlesource.com/c/sdk/+/124480 which caused the regression).

Original change's description:
> [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>

TBR=rmacnak@google.com,alexmarkov@google.com,asiva@google.com,regis@google.com

Change-Id: If8b2671f3cf4b0d080ba9f0472738326fec701d3
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/125100
Reviewed-by: Siva Annamalai <asiva@google.com>
Commit-Queue: Siva Annamalai <asiva@google.com>

Author: a-siva

runtime/vm/object.cc

@@ -4351,16 +4351,11 @@ 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::kLegacy);
+                   nullability);
   type ^= ClassFinalizer::FinalizeType(*this, type);
   set_declaration_type(type);
-  return type.ToNullability(nullability, Heap::kOld);
+  return type.raw();
 }
 
 void Class::set_allocation_stub(const Code& value) const {