return value of assignment expression

[bobzhang]

let f x = 
  x##hey#= 0
let () = f [%raw {| {hey : 3} |}]          

Generated code:

function f(x) {
  return x.hey = 0;
}
var x = ( {hey : 3} );
x.hey = 0;

Two questions:

1. shall we enforce the return type of assignment to be unit? or just assignment expression
2. if enforced to be unit, the generated code would be

function f (x){
  x.hey  = 0
  return 0 /* unit */
}