Native classes implementing interfaces with overlays triggerred a compiler
error due to the fact that overlays in native intefaces are "default"
methods and that the strategy of implementing them consists in creating
the corresponding synthetic override in classes that don't explicitly
implement the method.

Bug: #9440
Bug-Link: https://github.com/gwtproject/gwt/issues/9440
Change-Id: I30a100c4d83f13df61fed57a54afb3b25a153250