Fix watch mode for external extensions
Fix watch mode for external extensions

Webpack fixed this bug in a non-exported class that we keep made a
"copy" of. I have included the fix in our copy of the class.

This addresses an error causing wtach mode to fail to work when
using it with external extensions (not in dev mode)

jupyter/jupyterlab,jupyter/jupyterlab,jupyter/jupyterlab,jupyter/jupyterlab,jupyter/jupyterlab