AppState should not be using functions as property names

[ckknight]

Given the following code:

function run() {
  function handler() { /* ... */ }
  AppState.addEventListener('change', handler);
  return function () {
    AppState.removeEventListener('change', handler);
  }
}

Because AppState is managing its listeners in an object, the handler above will use its .toString() value as the key of listeners, which is equivalent between function calls.

One solution would be to use a Map, thus removing the implicit [[ToString]].