React should recognize the `valueAsDate` property on DOM elements

The valueAsDate property on the ES6 definition of HTMLInputElement is not yet recognized by React. It should be, though.

The valueAsDate property relieves the pain of having to deal with both browsers that support HTML5 date inputs and those who don't; the former require the value property to be of the form "YYYY-MM-DD" and the latter require a nicely formatted date (e.g. "MM/DD/YYYY") since the user will see exactly that. (React won't even allow that in conjunction with type="date", though, and proclaim that the machine readable format should be used.)
The browsers that do support HTML5 date input (should) alternatively support the date being passed in through the valueAsDate property. If we use that, we can always put a nicely formatted date in the value property. Pain relieved! (And also React should allow any format in the value property in that case.)

As indicated above, browser support is still lacky at best, even with the most avant garde browsers. Hence this is not an urgent issue for React. It will get relevant, though, and is herein duly noted.

The issue identifies HTMLInputElement, valueAsDate, value, and type="date" as the relevant entry points. First verify current behavior across browsers and React's handling of these properties; done means valueAsDate is recognized and the stated value-format restriction is resolved without breaking unsupported browsers.