announce when invalid characters are blocked

[notwaldorf]

Fixes https://github.com/PolymerElements/iron-input/issues/57.

We announce two messages:

• "Invalid character x not entered" whenever you're typing
• "Invalid string of characters not entered" whenever you paste something or manually call _onInput. The reason why this is different is that figuring out what the characters entered were is actually a bit expensive: you only have the string before you called _onInput , and the one after, and you would have to manually diff them which isn't trivial (if you paste in the middle of the string, you have to manually go through each character and figure out where the differences are). I'm not sure this is worth it, especially since reading that afterward is weird and potentially super verbose ("oops i accidentally pasted this entire tweet, sigh"). LMK if that doesn't sound reasonable @lpalmaro.

[valdrinkoshi]

Alright, tested on Android talkback: Invalid string of characters not entered gets repeated 4 times. In IE-Edge the text doesn't get announced, in IE11 all looks good

[notwaldorf]

@valdrinkoshi Does anything get announced in Edge? Like, say, paper-toast? These all sound like potential announcer bugs :(

[valdrinkoshi]

No, in fact i just opened an issue here https://github.com/PolymerElements/iron-a11y-announcer/issues/16 I'm checking if the multi-time announce is also a problem of the announcer

[valdrinkoshi]

Yep, also the multi-time announce is a problem of iron-a11y-announcer https://github.com/PolymerElements/iron-a11y-announcer/issues/17

This PR LGTM :+1:

[notwaldorf]

Sweet, thanks!