Remove updateAccessiblePeers

[zepumph]

from #814.

[zepumph]

@jessegreenberg I moved a lot of code from Accessibility.js over to AccessiblePeer.js. So far this is feeling very straight forward and nice. Please review.

[jessegreenberg]

@zepumph this looks really nice and is in the spirit of MVC. Also, addAssociationImplementationForAttribute is much improved, thanks!

I am seeing checked=false on lots of non input PDOM elements now. Probably because checked is now added with peer.setAttributeToElement() rather than directly with the setter like peer.primarySibling.checked = false. Maybe we need a function in AccessiblePeer just for checked?

Also, in the replacements of updateAccessiblePeers, I am seeing lots of checks like
peer && peer.doStuff().
At those locations is it possible for peer to not be defined?

[zepumph]

I am seeing checked=false on lots of non input PDOM elements now. Probably because checked is now added with peer.setAttributeToElement() rather than directly with the setter like peer.primarySibling.checked = false. Maybe we need a function in AccessiblePeer just for checked?

I think checked may need more work that just that, for example, see
AquaRadioButton.js (also in RadioButtonGroupMember):

    // a11y - Specify the default value for assistive technology. This attribute is needed in addition to
    // the 'checked' property to mark this element as the default selection since 'checked' may be set before
    // we are finished adding RadioButtons to the containing group, and the browser will remove the boolean
    // 'checked' flag when new buttons are added.
    if ( property.value === value ) {
      this.setAccessibleAttribute( 'checked', 'checked' );
    }

Perhaps we need to rethink how we are handling checked to begin with. @jessegreenberg would it be alright to create another issue for this, and address it after #814? As I think much may change in AccessiblePeer due to that issue, and we may have better insight into how we should proceed.

[zepumph]

I created a separate issue. Closing