The new self-contained widget is slow in google colab

[jleibs]

Our new rerun_notebook widget is significantly slower to load in colab than the old iframe approach.

Sample notebook demonstrating the issue:
https://colab.research.google.com/drive/1VjQsSHvhfsnQx8TEClTc7L0vDH6Fub-l#scrollTo=oI__Aa8JTpK9

In general it takes 6-8 seconds to load, even on a 2gig fiber internet connection.

For some reason we spend significant time waiting for the server to send to the bundled js asset:

Also, it seems like the way in which the tunneling happens possibly prevents things from being cached properly.

[jleibs]

I'm curious if this will require an approach where we try to load an http-hosted file, and if that fails then we fall back on something fetched from the anywidget module?