I have the same issue as in https://community.powerbi.com/t5/Developer/Developer-Visual-is-missing/td-p/207939 but it doesn't work and I do it on Power BI online site.
(I downloaded https://github.com/Microsoft/PowerBI-visuals-sampleBarChart/tree/master/assets. Ran npm install, pbiviz start and it claims
info Building visual...
done build complete
info Starting server...
info Server listening on port 8080.
and I made sure Enable developer visual for testing is enabled but that icon won't show up when I open a PowerBI report for edit on https://app.powerbi.com. I do it in my private workspace.)
Any idea please?