final ImmutableList<String> deferredTypeNames() {
    return ImmutableList.copyOf(deferredTypeNames.keySet());
  }
Qualified names of {@code @AutoValue} (etc) classes that we attempted to process but had to
abandon because we needed other types that they referenced and those other types were missing.
This is used by tests.
value/src/main/java/com/google/auto/value/processor/AutoValueishProcessor.java