The `bin/internal/engine.version` file controls which version of the Flutter engine to use.The file contains the commit hash of a commit in the <https://github.com/flutter/engine> repository.That hash must have successfully been compiled on <https://build.chromium.org/p/client.flutter/> and had its artifacts (the binaries that run on Android and iOS, the compiler, etc) successfully uploaded to Google Cloud Storage.
The `/bin/internal/engine.merge_method` file controls how we merge a pullrequest created by the engine auto-roller. If it's `squash`, there's only onecommit for a pull request no matter how many engine commits there are insidethat pull request. If it's `rebase`, the number of commits in the framework isequal to the number of engine commits in the pull request. The latter methodmakes it easier to detect regressions but costs more test resources.