[dmd-concurrency] Smoke test

[Walter Bright]

Steve Schveighoffer wrote:
> The only caveat is, we would *need* to change the semantics of shared so that you can have a non-shared reference to a shared class instance easily.

That's the so-called "tail-shared" problem. We tried (hard) to make 
"tail-const" work, where the reference is mutable but the contents were 
const. It sounds simple, but just does not work in a language with 
implicit reference semantics.