dillon1000/react

Workaround serious Babel bug

I ran into the same issue that @poteto and @gsathya (and probably @mofeiZ) have 
run into: "Duplicate declaration of '$'" caused by Babel visiting a function 
twice despite our calling `skip()`. This PR keeps a set of nodes that we have 
already visited to avoid visiting them again, as a workaround for skip not 
working. 

# Test Plan 

Synced to www and confirmed that the previous bug no longer reproduces, and the 
compiled output looks sane.