js/lab fails to build with Jupyter Lab 1.2.9 + master

I am trying to do a local build in my Dockerfile to pull in beakerx 1.5.0, and while the beakerx modules all work, the labextension fails to build with an error about a bad reference to ../lib/tree.js in dist.

```Dockerfile
WORKDIR /app/beakerx/js/lab
RUN jupyter labextension install . --no-build

WORKDIR /app/beakerx/js/lab-theme-dark
RUN jupyter labextension install . --no-build

WORKDIR /app/beakerx/js/lab-theme-light
RUN jupyter labextension install . --no-build

RUN jupyter lab build
```

The error is below:

> [LabBuildApp] > node /usr/local/lib/python3.8/site-packages/jupyterlab/staging/yarn.js run build:dev:minimize
[LabBuildApp] yarn run v1.15.2
$ jlpm run build:dev
$ jlpm run build
$ webpack
ModuleNotFoundError: Module not found: Error: Can't resolve '../lib/tree.js' in '/usr/local/share/jupyter/lab/staging/node_modules/beakerx-jupyterlab/dist'