Following the rules in the algorithm from
https://webidl.spec.whatwg.org/#js-platform-objects, "To Internally
create a new object implementing the interface interface", this change
incorporates the steps to load a prototype from new.target, and write
it to the created instance returned from constructor_impl(). This
mirrors the code for generate_html_constructor(), which incorporates
additional steps needed by Custom Elements.
Bug #334