Only load istanbul plugin in development mode

What does this MR do?

Conditionally applies the istanbul babel plugin only in development.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

changed title from only load istanbul plugin in development mode to Only load istanbul plugin in development mode

marked the task Conform by the merge request performance guides as completed

marked the task Conform by the style guides as completed

marked the task Branch has no merge conflicts with master (if it does - rebase it please) as completed

marked the task Squashed related commits together as completed

added ~155980 frontend ~12980 labels

/cc @marin this should fix the omnibus build failure. A development tool was accidentally added to the production config.

I've tested this out with NODE_ENV=production bundle exec rake gitlab:assets:compile and it passed.

The bug was introduced in !9133 (merged)

assigned to @alfredo1

changed milestone to %8.17

I can confirm de fix running webpack-prod locally and it passes now. Thanks @mikegreiling LGTM

enabled an automatic merge when the pipeline for 23e6cd6f succeeds

merged

I've tested this out with NODE_ENV=production bundle exec rake gitlab:assets:compile and it passed.

This is the second time that I broke this because I only tested my changes in dev environment. Could we make this call part of CI?