I was expecting `tree.isGenerator()`.
It looks like we could pass two flags to this function, `transformOptions.generators && isGenerator(functionKind)` and `transformOptions.asyncFunctions && isAsync(functionKind)` in place of `functionKind` then compute them in the caller as eg `transformOptions.asyncFunctions && tree.isAsync()`, avoiding the two helper function `isGenerator()` and `isAsync()`.