input[type="datetime-local"], the $valid property not set to true on subsequent clearing with button

[piotr-dobrogost]

I'm submitting a ...

• [x ] bug report

Current behavior:
When using input[type="datetime-local"] directive without required attribute then clearing input with clear button (blue cross icon) sets $valid property to true only the first time the button is clicked. Subsequent clicks do not set the property to true value anymore.

Expected / new behavior:
Clicking clear button of a control for non-required input[type="datetime-local"] directive should always set $valid property to true.

Minimal reproduction of the problem with instructions:
https://plnkr.co/edit/eFypeLyTVuBi394KhKwR?p=preview
This is version of plunkr from the docs https://docs.angularjs.org/api/ng/input/input%5Bdatetime-local%5D with two modifications: initial Date object is null and attribute required is removed from input[type="datetime-local"].

1. Enter invalid date (only month for example)
   myForm.input.$valid == false
2. Click clear button (blue cross icon)
   myForm.input.$valid == true
3. Enter invalid date (only month for example)
   myForm.input.$valid == false
4. Click clear button (blue cross icon)
   myForm.input.$valid == false <- should be true instead

AngularJS version: v1.6.10-build.25885+sha.c68b31c (current snapshot)
Browser: Chrome 65.0.3325.162
OS: Fedora 28

[gkalpak]

Closing as duplicate of #14740. It is partly a Chrome issue/idiosyncracy. We could work around it inside AngularJS, but you can work around it in your app too (see #14740 discussion and specific suggestions).

If anyone wants to work on a PR, they are more than welcome 😃