$key doesn't support dot property accessors

[Bartozzz]

In my opinion $key should work with both bracket (e.g. foo["bar"]) and dot (e.g. foo.bar) notations.

Reproduction on Flow/try:

/* @flow */

type Properties = {
  a: string,
  b: string
};

class Foo<Properties: Object> {
  $key: $Keys<Properties>;
  $value: $Values<Properties>;
};

const foo: Foo<Properties> = new Foo();

// ✔ Will work fine:
foo["a"] = "adsasd";

// ✔ Will throw an error because "c" in not in Properties:
foo["c"] = "adsasd";

// ✘ Will throw an error because of the dot notation:
foo.b = "adsasd";

// ✘ Will thrown an error because of deconstruction:
const {a, b} = foo;

(nothing changes when Foo extends Object)