All extensions are tested with an Observable implementation based on zen-observable.
zen-observable is not directly used in order to guarantee that no extension is using any of the zen-observable additional features which are not part of the Observable proposal.
Some tests depend on extensions, thus they are designed incrementally in order to avoid an issue hidden by cumulative bugs. The following graph displays the precedence between test files and not between the extensions themselves:
Arrows should be interpreted as "relies on a feature tested by".