code: private boolean hoistIfExtern(CompilerInput input) {
      if (input.getHasExternsAnnotation()) {
        // If the input file is explicitly marked as an externs file, then move it out of the main
        // JS root and put it with the other externs.
        externsRoot.addChildToBack(input.getAstRoot(this));
  
        JSModule module = input.getModule();
        if (module != null) {
          module.remove(input);
        }
  
        externs.add(input);
        return true;
      }
      return false;
    }
docs: Hoists a compiler input to externs if it contains the @externs annotation.
  Return whether or not the given input was hoisted.