Thanks for your answer Eric.
In our project, we load all global javascript files in the root level "index.html" file, which also imports the element.html file which includes all the components related import. Then we assume that all components can safely use the variables in those global javascript files. Plus, all the non-polymer codes can also use those global variables.
It's working so far, but I wonder if you see hidden trap of this approach. One thing I'm not sure is, since element.html is imported in the head, while all the global javascript files are loaded in the end of body, could it be possible that when Polymer components are ready, some of the js files are not loaded yet, hence the error of undefined variable could happen?
Thanks in advance!