Revert "[vm] Normalize FutureOr<T?> to FutureOr<T?>?."

This reverts commit d0a1404.

Reason for revert: Doesn't follow declared type normalization rules in spec.

TEST=Revert

Original change's description:
> [vm] Normalize FutureOr<T?> to FutureOr<T?>?.
>
> Previously, FutureOr<T?>? was normalized to FutureOr<T?>, since the two
> types are equivalent: FutureOr<T?> is the union of T? and Future<T?>,
> and so is nullable since T? allows null. However, this means there are
> nullable AbstractTypes where nullability() returns kNonNullable.
>
> Instead, normalize FutureOr<T?> to FutureOr<T?>?, so that checking the
> nullability of the outer type is sufficient to determine nullability.
>
> TEST=vm/dart/regress_48522
>
> Fixed: #48522
> Change-Id: I710facc84ec79485716ee543c9d272f59f87106c
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/237061
> Reviewed-by: Martin Kustermann <[email protected]>
> Commit-Queue: Tess Strickland <[email protected]>

[email protected],[email protected]

Change-Id: I6999963d907b9ca79d348aca85bf8949e06471b6
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/237081
Reviewed-by: Martin Kustermann <[email protected]>
Commit-Queue: Tess Strickland <[email protected]>

Author: sstrickl

runtime/tests/vm/dart/regress_48522_test.dart

@@ -19847,14 +19847,8 @@ AbstractTypePtr AbstractType::NormalizeFutureOrType(Heap::Space space) const {
       ASSERT(object_store->nullable_future_null_type() != Type::null());
       return object_store->nullable_future_null_type();
     }
-    // To avoid having to special case FutureOr in nullability checks, we lift
-    // the nullability of the type argument to FutureOr as appropriate.
-    if (IsNullable() || unwrapped_type.IsNullable()) {
-      // FutureOr<T?> = FutureOr<T?>* = FutureOr<T?>?, so mark as nullable.
-      return Type::Cast(*this).ToNullability(Nullability::kNullable, space);
-    } else if (unwrapped_type.IsLegacy()) {
-      // FutureOr<T*> = FutureOr<T*>*, so mark as legacy.
-      return Type::Cast(*this).ToNullability(Nullability::kLegacy, space);
+    if (IsNullable() && unwrapped_type.IsNullable()) {
+      return Type::Cast(*this).ToNullability(Nullability::kNonNullable, space);
     }
   }
   return ptr();