+str #16038 Create Source/Flow/Sink from partial flow graphs

Fixes #16038
Here it is known that the builder must contain one UndefinedSink and one UndefinedSource, and it should be possible to call `builder.toFlow` with those. You mean that the `I` and `O` might not match corresponding types in the undefined things? Does that matter? It will not run until the undefined are attached?