[vm/ffi] No error if Pointer type is a top type

[sgrekhov]

The following code works without any issues

import "dart:ffi";
import 'package:ffi/ffi.dart';

void main() {
  Pointer<Int16> p = calloc<Int16>();

  Pointer<dynamic> p1 = Pointer.fromAddress(p.address);
  Pointer<void> p2 = Pointer.fromAddress(p.address);
  Pointer<Object?> p4 = Pointer.fromAddress(p.address);
  Pointer<Never> p5 = Pointer.fromAddress(p.address);
}

For the comparison, Array class produces errors for all of the types mentioned above

class MyStruct extends Struct {
  @Array(16)
  external Array<void> a2;
//               ^^^^
// [analyzer] unspecified
// [cfe] unspecified

  @Array(16)
  external Array<dynamic> a3;
//               ^^^^^^^
// [analyzer] unspecified
// [cfe] unspecified

  @Array(16)
  external Array<Object?> a5;
//               ^^^^^^^
// [analyzer] unspecified
// [cfe] unspecified

  @Array(16)
  external Array<Never> a6;
//               ^^^^^
// [analyzer] unspecified
// [cfe] unspecified
}

I believe, Pointer class should produce error for these types as well

Tested on Dart SDK version: 2.16.0-edge.3d1f6a48b79e2cc5a4ee9d2ea3a7a385a3b33eaa (be) (Thu Nov 18 00:58:40 2021 +0000) on "windows_x64"

cc @dcharkes