improve the typing of onCompletedSucessfully

[lemke-ethan]

there is a problem with unions -- it types the onCompletedSucessfully argument as a union of two functions rather than as a function with a union argument. i.e., the render functions becomes (onCompletedSucesfully:TypeA)=>{} | (onCompletedSucesfully: TypeB)=>{} rather than (onCompletedSucesfully:TypeA|TypeB)=>{}.