commit(s) to merge: 86f3fde

merge instructions: https://dart-review.googlesource.com/c/sdk/+/149420

reason: This CL fixes a problem where persistent front-end process used for incremental compilation during development would leak more and more memory with each recompilation. The amount of leaked memory depends on which parts of their app user is editing. As an experiment editing 20 files in flutter framework and reloading after each caused FE server to leak 3GB. This is believed to be the cause of some reports in #41862

/cc @franklinyow