querySelector return type could be more specific for single-element selectors

[jelbourn]

The return types for querySelector and querySelectorAll (in the lib.es6.d.ts) could provide more specific return type information when the selector a single element selector for a known element type.

I.e., right now the types are

interface NodeSelector {
    querySelector(selectors: string): Element;
    querySelectorAll(selectors: string): NodeListOf<Element>;
}

But it could be expanded in the style of getElementsByTagName:

interface NodeSelector {
    querySelector(selectors: "a"): HTMLAnchorElement;
    querySelector(selectors: "abbr"): HTMLPhraseElement;
    ...
    querySelector(selectors: string): Element;
}

It's a minor convenience, but does eliminate the occasional typecast.

[DanielRosenwasser]

I don't think this is totally unreasonable. @mhegazy @zhengbli any thoughts on this?

[mhegazy]

PRs welcomed. here is some instructions on submitting lib.d.ts PRs: https://github.com/Microsoft/TypeScript/blob/master/CONTRIBUTING.md#contributing-libdts-fixes