dillon1000/react

Support for with empty update expression

Adds support for `for` statements with an empty or unreachable update 
expression. In both cases, reversePostorderBlocks() will remove the 
empty/unreachable update block, leaving the ForTerminal.update pointing to a 
non-existent block. We explicitly rewrite this (much like we null out 
unreachable fallthroughs after shrink). When transforming to ReactiveFunction, 
we emit the update block as null if it was the same as the test block.