Datepicker placeholder only clears after triggering its change detection (by clicking somewhere)..

Bug, feature request, or proposal:

If you have a basic datepicker with floatLabel="never", its placeholder is not cleared on keyboard input. Only happens with invalid input ("abcd"), but it can be quite annoying because, if you are validating with some format like DD/MM/YY, the placeholder will only go away when you type exactly that format.

(Check stackblitz for a working example)

• Placeholder overlaying input text:

What is the expected behavior?

Placeholder to be cleared on keyboard input.

What are the steps to reproduce?

Here is just a fork from the basic datepicker from documentation with added floatLabel="never". You should type an invalid date like "abc".

https://stackblitz.com/edit/angular-pr5nuh

Which versions of Angular, Material, OS, TypeScript, browsers are affected?

Angular: 6.1.3
Material 6.4.6
Chrome 69 @ OSx

Awesome work @crisbeto

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