We have many instances of this object and we clone them as well (e.g. ng-repeat).
This should save some memory and performance as well.
Double prefixed private properties of attr object:
attr.$element -> attr.$$element
attr.$observers -> attr.$$observers
Update shallowCopy to not copy $$ properties and allow passing optional destination object.
The `attr` object was only shallow copied which caused all observers to be shared.
Fixing similar issue in ng-* boolean attributes as well as ng-src and ng-href.