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In https://github.com/flutter/engine/pull/56844 we saw a 15% increase in performance by switching to absl::flat_hash_map. I suspect we say see even better results ([source](https://martin.ankerl.com/2022/08/27/hashmap-bench-01)). The absl guidance is to default to using this. test exempt: no intentional functional change, just performance [C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style