Minor DOM FragmentInstance refactors

[jackpope]

Handles TODOs, small follow up refactors

[react-sizebot]

Comparing: 90b2dd4...4fd0d40

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-stable/react-dom/cjs/react-dom.production.js	=	6.84 kB	6.84 kB	=	1.88 kB	1.88 kB
oss-stable/react-dom/cjs/react-dom-client.production.js	+0.07%	609.58 kB	609.99 kB	+0.06%	107.80 kB	107.87 kB
oss-experimental/react-dom/cjs/react-dom.production.js	=	6.84 kB	6.84 kB	=	1.88 kB	1.88 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js	+0.06%	675.51 kB	675.92 kB	+0.06%	118.75 kB	118.82 kB
facebook-www/ReactDOM-prod.classic.js	+0.06%	695.14 kB	695.58 kB	+0.07%	122.19 kB	122.28 kB
facebook-www/ReactDOM-prod.modern.js	+0.06%	685.52 kB	685.97 kB	+0.07%	120.59 kB	120.67 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

Generated by 🚫 dangerJS against 4fd0d40