[vm/ffi] struct inheritance

[dcharkes]

Should we allow subtyping of structs?

import dart:ffi show Struct, Int8;

A extends Struct {
  @Int8()
  int x;
}

B extends A {
  @Int8()
  int y;
}

Where B has all the fields of A.

/cc @sjindel-google

[sjindel-google]

I don't see a reason to allow it -- it's not widely defined in native ABIs.

[sjindel-google]

We won't support it since you can't extend structs in C. Also, it's semantically the same as composition.