For classes which implement the `ArrayAccess` interface, such as the `ArrayObject` class, using `$this['key']` is a perfectly fine, working syntax and unsetting an item in the object using that syntax will still work and is unaffected by the changes in PHP 7.1.

The sniff, however, would throw a false positive for this.

This PR fixes that.

Includes unit tests.

Fixes 780

wimg approved these changes Dec 23, 2018