Make a few DOM properties nullable and IDL classes iterable

[loganfsmyth]

These DOM properties are definitely nullable, though in the average case they probably exist. I assume it's best for the type definitions to cover all the possibilities.

For the iterability, Chome added these in a few releases ago and the next release of core-js should include polyfill logic for them. More info in zloirock/core-js#249

[gabelevi]

I find the argument made in the OP of zloirock/core-js#249 convincing. Definitely cool marking these as Iterable.

Other changes look good to me too! I'll get this merged! Thanks for the contribution!

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[OlegLustenko]

How to use this syntax now ?

const body: HTMLElement = document.body;
// [flow] [flow] null (This type is incompatible with HTMLElement)

const body: HTMLElement | null = document.body;
body.addEventListener('mousemove', event, false);
// [flow] [flow] call of method `addEventListener` (Method cannot be called on possibly null value)

Should I'm run this way now ?

const body: HTMLElement | null = document.body;

From the documentation:

/* @flow */
var o: ?string = null;
if (o == null) {
  o = 'hello';
}
print(o.length);

Do I need to perform a null check?
What If I'm sure my code will execute when the body isn't null?

[loganfsmyth]

You'd have to perform a null check, and either handle that case specifically, or throw so Flow knows that the value is guaranteed to exist after that point.