It allows for querying on fields on un-indexed super-classes (see `DSLTest#testPolymorphicQueryWithKeywordTermForUnindexedSuperTypeReturnsIndexedSubType()`).

It's really a trade-off, but @emmanuelbernard seems to prefer the new approach. The idea is that it's made sure beforehand that any queried fields exist. This will be the case for the parser, but of course not when using the DSL directly.