fix(components/checkbox): focus classes are not being applied to mat-…

[maxim-marchenko-bp]

…checkbox

Fixes the issue in Angular Material 'checkbox' component where classes related to focusing mechanism weren't applied. Related to missing FocusMonitor usage which is being used in other components.

[crisbeto]

The focus monitor is an internal implementation detail, it's not meant to be applied to all of the components.

[maxim-marchenko-bp]

@crisbeto I've faced this when tried using the, for example cdk-keyboard-fosed class, but it's missing for this component. I need to perform specific things depending on focusing mechanism, and found that it's applied for button, radio-button, but no the checkbox. Is there a reason that this was cut for this component, and I should find a workaround on my app?

[crisbeto]

We introduced the focus monitor a long time ago when the only way to target focus was :focus. These days :focus-visible should provide similar functionality (https://developer.mozilla.org/en-US/docs/Web/CSS/:focus-visible). I don't feel too strongly about this, but we just never treated the focus monitor classes as a public API. cc @andrewseguin