Move to using ES6 source code

[keego]

#214 kickstarted the move to ES6, this issue intends to help the transition complete smoothly by outlining the transition process:

Steps to complete transition

• [x] 1. Added an ES6 build target to develop by transpiling to ES6 then bundling with rollup.
  • Accomplished by #214
• [x] 2. Bring in an ES6 only branch, mirroring develop, providing an ES6 branch for new PRs to target
  • Accomplished #229
• [x] 3. Finish merging any outstanding PRs that target develop (making sure to merge transpiled ES6 changes from each PR into the develop-es6 branch as we go)
  • Waiting on #208 and #234 to be merged or retarget develop-es6
• [ ] 4. Merge develop-es6 back into develop (develop is ES6 only from here on out) and resume targeting develop as the default branch
• [ ] 5. Finish merging any outstanding PRs that targeted develop-es6 (making sure to also merge changes from each PR into the develop branch as we go)
• [ ] 6. Remove develop-es6 branch after no more PRs target it

Why so many steps?

This approach attempts to prevent clobbering existing work being done through the transition by providing a new ES6 branch for new PRs to target and, more importantly, allow developers to stop targeting the non-ES6 branch so it can be closed.

[Rayman]

Hi, what is the status of the ES6 migration? I'm really interested because ES6 imports mean for me a reduced bundle size by tree shaking.

I see that there are already migration scripts to convert the codebase to src-esm. Are we now at step 4?