@JsFunction interfaces were not completely loaded by UnifyAst
if the functional method was never called. This fact caused that
the interface was not considered as implementable externally, which
in turn confused restriction checker ending in a spurious warning.

Bug: #9376
Bug-Link: https://github.com/gwtproject/gwt/issues/9376
Change-Id: I92870fde72ab864bc42db8355e283ef3fe17d2ca