Error: unknown argument: '-enable-experimental-concurrency`

I have latest toolchain(snapshot 26/04) installed in my MacBook. And I am trying to add flags -Xfrontend -enable-experimental-concurrency. But when running Xcode is giving error <unknown>:0: error: unknown argument: '-enable-experimental-concurrency' Command CompileSwift failed with a nonzero exit code

This is not happening for 09/03 snapshot. It is compiling without any error.
Why experimental concurrency is not available on latest snapshot?