Ensure Flutter compatibility

[greenrobot]

It was brought up on reddit that there might at least one compatibility issue with Flutter:

I noticed in the box class you're using dart:mirrors. As far as I know that doesn't work in flutter, did that recently get changed? Here is some discussion about it from a while ago.

So, ensure everything run on Flutter.

Bonus task: can we unit test using Flutter?

[greenrobot]

mirrors affects the following reflection:

    var ret = reflectClass(T).newInstance(Symbol.empty, []);

and

    var instRefl = reflect(inst);

Two options:

• https://github.com/dart-lang/reflectable
• Getting our own code gen ready

If reflectable is a quick fix, then let's go for it. We'll have our code gen either way in the future, but it's a bigger task.

[nalenz-objectbox]

The binding doesn't use any reflection now anymore, but code generated using source_gen. Because we want to use Flutter on desktop, only Dart >=2.5 is supported, so only the branch dev-2.5 contains the respective example.