[2.0.0-beta1] "Data from field" source plugin avaiable for UnknownPropType

I see folowing possible solutions:

• In SourcePluginManager::getDefinitionsForPropType: if ($prop instanceOf UnknownPropType) {return [];}
• Add prop types to EntityFieldSource via alter hook and make prop_type required.
• Add a is isApplicable method to the source. (We had something like this before. :))

@just_like_good_vibes: I need your 5 cents here :)

my question, why do we need unknownPropType ? sorry i forgot.
if it is an hardcoded edge case, where no sources should be shown,
my favorite solution between the three solutions proposed would be : "In SourcePluginManager::getDefinitionsForPropType: if ($prop instanceOf UnknownPropType) {return [];}"

I added the if.

Pierre, would you review please :)?