code: @Override
    public CompilerInput getInput(InputId id) {
      // TODO(bradfordcsmith): Allowing null id is less ideal. Add checkNotNull(id) here and fix
      // call sites that break.
      if (id == null) {
        return null;
      }
      return inputsById.get(id);
    }
docs: interface, and which ones should always be injected.