code: private void markExterns(ImmutableList<CompilerInput> originalInputs) {
      for (CompilerInput input : originalInputs) {
        if (input.getHasExternsAnnotation()) {
          input.setIsExtern();
        }
      }
    }
docs: Marks inputs with the @externs annotation as an Extern source file type.
  This is so that externs marking can be done before dependency management,
  and externs hoisting done after dependency management.