Improve chunk names in dev mode

Created by: jerstucki

Description

Names of dynamically imported chunks are weird in dev mode. E.g.

• 1.static/js/bundle.js
• 2.static/js/bundle.js

It's not a big deal but they all show up as bundle.js in Chrome's network inspector.

Production builds are fine

Expected behavior

The chunk names should be prepended to the file name.

• static/js/1.bundle.js
• static/js/2.bundle.js

Environment

react-scripts@0.10.0-alpha.328cb32e