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`kDefaultBackendType` is intended to make life easier for authors of tests, but in any switch statement where it's used (currently just a single location), we rely on ordering it first and `#ifdef`ing out all backends that aren't available. Instead, we define a static function that returns the default that callers can invoke instead. This avoids relinace on case ordering and fallthrough. In https://github.com/flutter/engine/pull/56722 we split backends out into separate translation units, and ideally should remove the `#ifdef`s, which means we can't rely on this trick anymore. [C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style