code: @Override
    public void process(Node externsRoot, Node jsRoot) {
      checkNotNull(scopeCreator);
      checkNotNull(topScope);
  
      Node externsAndJs = jsRoot.getParent();
      checkState(externsAndJs != null);
      checkState(externsRoot == null || externsAndJs.hasChild(externsRoot));
  
      if (externsRoot != null) {
        check(externsRoot, true);
      }
      check(jsRoot, false);
    }
docs: Main entry point for this phase of processing. This follows the pattern for
  JSCompiler phases.
  
  @param externsRoot The root of the externs parse tree.
  @param jsRoot The root of the input parse tree to be checked.