stage 2 assigned reviewer sign-off

[michaelficarra]

As a formality, just getting written sign-off in this issue from the assigned reviewers (necessary for the planned advancement to stage 3 at the next meeting)

• [x] @ljharb
• [x] @rwaldron
• [ ] @spectranaut

[ljharb]

Signed off! The spec steps work well in both of my polyfills as well:

• https://github.com/es-shims/Array.prototype.flatten
• https://github.com/es-shims/Array.prototype.flatMap