code: private boolean isGoodRefToAdd(@Nullable StaticRef ref) {
      return ref != null
          && ref.getNode() != null
          && ref.getNode().getStaticSourceFile() != null
          && !Compiler.SYNTHETIC_EXTERNS.equals(ref.getNode().getStaticSourceFile().getName());
    }
docs: Helper for addSymbolsFrom, to determine whether a reference is acceptable. A reference must be
  in the normal source tree.