In assignments, property keys should be evaluated only after checking the object is not null

[andreabergia]

This should throw a TypeError because we are accessing null, but instead it is throwing a MyError:

function MyError() {}
MyError.prototype.toString = () => 'MyError';

var base = null;
var prop = {
    toString: function() {
        throw new MyError();
    }
  };
 
base[prop] = 42;