fix(tabs): update mat-tab-link to set aria-current when active

[stevenyxu]

mat-tab-header already does this:
https://github.com/angular/material2/blob/abc3d38c57146443c848d5ba26fd2fab8ca185d6/src/lib/tabs/tab-group.html#L11

And you can see aria-selected is being set properly on the main tabs demo page
https://material.angular.io/components/tabs/overview.

But mat-tab-nav-bar (on the same page, but no live demo) doesn't set aria-selected at all.

[tinayuangao]

LGTM.

The tab-link should have role (#11410) and aria-selected.

[stevenyxu]

Thank you! Rebased and ready for further review.

[stevenyxu]

Updated from aria-selected to aria-current per discussion on #11410. nav > link pattern should use aria-current to indicate the current active nav link.

[jelbourn]

LGTM

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[stevenyxu]

Per offline discussion, Angular Material is not interested in this fix for now pending further analysis. Users are expected to implement their own indicator on which of the mat-tab-link is active. A good candidate seems to be https://www.w3.org/TR/wai-aria-1.1/#aria-current.

[stevenyxu]

Reopening since it looks like this is the way we want to go.

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