[Reviewer Andy] Dynamic Sproutlet loading

Andy

Can you review the changes for dynamic sproutlet loading.  I'll email updated spec separately.

Mike
This is existing code (just flagged as new because it has moved).  I think the rationale here is that, the usual use case for running debug is that you've just run a test that has failed and you want to debug it, and this means you don't have to wait for tedious dependency checking to happen before the debugger starts.  I'd rather not change it as I assume it's been done for a reason.