fix ($compile): keep prototype properties for template URL directives

Previously, if a directive definition object was defined with methods like `compile`
provided on the prototype rather than the instance, the Angular compiler failed
to use these methods when the directive had a `templateURL`. This change ensures
that these prototypical methods are not lost.

This enables developers to define their directives using "classes" such as
in CoffeeScript or ES6.

Closes #10926