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issue: https://github.com/flutter/flutter/issues/159745 This tweaks ReactorGLES so that one can easily opt out of it's mutex/hash map for cases where we can statically reason about the safety of doing so. The goal here was to make migration of existing code really easy to do. It may be in the future that everything is an untracked handle? We can move there in baby steps. Potential follow up PRs: - Move `Pipeline` to use untracked handles - Move `DeviceBufferGLES` to use untracked handles - Add a new method to synchronously delete untracked handles - Start storing handles to be deleted in its own vector, so handles_ doesn't need to be used for deleting untracked handles ## Pre-launch Checklist - [x] I read the [Contributor Guide] and followed the process outlined there for submitting PRs. - [x] I read the [Tree Hygiene] wiki page, which explains my responsibilities. - [x] I read and followed the [Flutter Style Guide] and the [C++, Objective-C, Java style guides]. - [x] I listed at least one issue that this PR fixes in the description above. - [x] I added new tests to check the change I am making or feature I am adding, or the PR is [test-exempt]. See [testing the engine] for instructions on writing and running engine tests. - [x] I updated/added relevant documentation (doc comments with `///`). - [x] I signed the [CLA]. - [x] All existing and new tests are passing. If you need help, consider asking for advice on the #hackers-new channel on [Discord]. <!-- Links --> [Contributor Guide]: https://github.com/flutter/flutter/blob/master/docs/contributing/Tree-hygiene.md#overview [Tree Hygiene]: https://github.com/flutter/flutter/blob/master/docs/contributing/Tree-hygiene.md [test-exempt]: https://github.com/flutter/flutter/blob/master/docs/contributing/Tree-hygiene.md#tests [Flutter Style Guide]: https://github.com/flutter/flutter/blob/master/docs/contributing/Style-guide-for-Flutter-repo.md [C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style [testing the engine]: https://github.com/flutter/engine/blob/main/docs/testing/Testing-the-engine.md [CLA]: https://cla.developers.google.com/ [flutter/tests]: https://github.com/flutter/tests [breaking change policy]: https://github.com/flutter/flutter/blob/master/docs/contributing/Tree-hygiene.md#handling-breaking-changes [Discord]: https://github.com/flutter/flutter/blob/master/docs/contributing/Chat.md