The problem with ClosureTestCase>>#constructFibonacciBlock is that it
has returns (^) that get executed after its home context has alread
returned, so it has no where to return to.  My error is that this should
raise a
cannotReturn: instead of blowing up on a dead frame.  I will fix this. 
If you remove the returns the test case will work.  If you want you can
change '^' to 'thisContext return:' to do a local return.

I posted Rob's and Boris's test case on the block closure page ("New
Block Closure Version").  Thanks, guys.

Cheers,
Anthony