Shady Parts: design for build step

[aomarks]

Investigate whether/how an optional build step could be used to improve performance of shadow parts support in shady css.

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale[bot]]

This issue has been automatically closed after being marked stale. If you're still facing this problem with the above solution, please comment and we'll reopen!