Currently, it is easy to accidentally define a Diagnostic subclass that doesn't actually implement the Diagnostic interface. This can break results.html. Presubmit should smoke-test all Diagnostic subclasses to ensure that they can be

created empty
cloned
merged
visualized
serialized
deserialized
tested for equality

Some of this is already done. Let's wait to finish it until after RelatedHistograms are gone.

From https://github.com/catapult-project/catapult/issues/3804