groovy-ant part of PR#456
GROOVY-6969: Set.flatten() should return Set
Merge pull request #493 from yukoba/SetFlatten

GROOVY-6969: Set.flatten() should return Set
GROOVY-6970: Type checker should be able to choose between two equivalent interface methods
GROOVY-6849: In case of ambiguity, type checker should be able to choose between an extension method and an interface method