Function returns should not be invariant

[aegershman]

see: https://rules.sonarsource.com/java/RSPEC-3516

When a method is designed to return an invariant value, it may be poor design, but it shouldn’t adversely affect the outcome of your program. However, when it happens on all paths through the logic, it is surely a bug.

[sambsnyd]

Going to close this for now. While we could hack something together now, a more thorough treatment requires support for dataflow analysis that we haven't yet begun to design.

[jkschneider]

@sambsnyd Dataflow analysis is in development.