[vm/ffi] No error in CFE if generative constructor of Union subtype is called

[sgrekhov]

The following tests fails in CFE

import "dart:ffi";

class U1 extends Union {
  @Int8()
  external int i;

  U1(this.i);
//        ^
// [analyzer] unspecified
// [cfe] unspecified
}

void main() {
  new U1(42);   // No error in CFE here. Analyzer reports an error here
//    ^^
// [analyzer] unspecified
// [cfe] unspecified
}

Instances of Union cannot be created via generative constructor, so new U1(42); must be an error in CFE as well

Tested on the edge version of the SDK