Assuming that we are watching structurally an array for changes (ex.: `["foo", "bar"]) and the reference changes to null I need to know, form the change detection system, that "foo" and "bar" were removed. As of today there is no such information delivered from the change detection system (changes got bot currentValue and previousValue equal to null) so ones would have to keep this state "manually".

Based on discussion with @vsavkin while working on the [class]="expr" directive.