extractChildren produces a flat child-node array for DOM patching. It hides fragment nodes while preserving the order of their descendants, including descendants nested inside other fragments. section 8.2.6
Implement it with three small responsibilities:
- When a child is a fragment, recursively extract its children and spread them into the result. section 8.2.6
Nested fragments
Given a node containing an ordinary child and a fragment whose contents include another fragment, extractChildren returns one flat array of ordinary descendants. patchChildren can then compare old and new arrays with arraysDiffSequence. section 8.2.6
