See this question on StackOverflow:
http://stackoverflow.com/questions/29785887/scripts-loading-out-of-order-in-chrome

I know you can use a loader like javascript-loaders to define the order that script tags are run. Potentially the same can be done here.

For now I'm moving the external script tag to my head and including it on every page of my app, so it should be guaranteed to be loaded before I reach the page-specific code.