[blocking release] es2017 migration breaks custom widget libraries written in es5

Now that we target es2017 in core packages, custom widget libraries that subclass WidgetModel with the es5 syntax don’t work anymore.

The issue is that static properties from the es6 bases are not visible to the derived classes.

Two approach are possible:

• move back to building to es5 in widgetsnbextension
• override Backbone.Model.extend in the base WidgetModel to properly polyfill the es6 extends including the behavior with respect to static properties.

[jasongrout]

I am working on a PR reverting the move to es2017.

+1 for the base and controls package. I think it’s probably not necessary to move the jlab manager back to es5.

[SylvainCorlay]

I am working on a PR reverting the move to es2017.