o(￣へ￣)o

replace if-else with switch-case in ConstraintInsetTarget and constraintOffsetTargetValue before: ```Swift if let amount = self as? Float { ... } else if let amount = self as? Double { ......

I tried this in playground: >let count = [1, 2, 3, nil, nil, 4, 5, 6, 7] > .flatten() and I got an error: >Playground execution failed: > >error: RbSwift.playground:1:6:...

sample code happened in **null safety**: ```dart _.compact(['a', null, 'c']).join() // => 'anullc' ```

weak property will get nil in an async callback, and it will cause crashes