fix(input): improve html5 validation support

This CL improves mocking support for HTML5 validation, and ensures that it
works correctly along with debounced commission of view values.

Closes #7936
Comments are good, but those comments turned out to be wrong, we need to revalidate even if there's a `valueMissing` --- although I think "badInput" is the only validation error that should report the value as the empty string... hmm