ImmutableList<Protoclass> protoclassesFrom(Iterable<? extends Element> elements) {
      return round().protoclassesFrom(elements);
    }
Default type adapters should only be called if {@code Gson.TypeAdapters} annotation is
definitely in classpath. Currently, it is called by for mongo repository module,
which have {@code gson} module as a transitive dependency.
@return default type adapters
value-processor/src/org/immutables/value/processor/meta/Proto.java