Overflow has been removed in Flutter master

[cbenhagen]

See https://github.com/flutter/flutter/pull/61366

I was not sure if you prefer to add the old Overflow enum to custom_stack.dart or to move everything to using clipBehaviour like in that change so I did not open a PR.

This results in build errors like this:

[+2140 ms] [+2138 ms] ../../../.pub-cache/hosted/pub.dartlang.org/flutter_storyboard-0.1.6/lib/src/custom_stack.dart:293:9: Error: Type 'Overflow' not found.
[        ] [   +1 ms]   final Overflow overflow;
[        ] [        ]         ^^^^^^^^
[        ] [        ] ../../../.pub-cache/hosted/pub.dartlang.org/flutter_storyboard-0.1.6/lib/src/custom_stack.dart:344:5: Error: Type 'Overflow' not found.
[        ] [        ]     Overflow overflow = Overflow.clip,
[        ] [        ]     ^^^^^^^^

[rodydavis]

Test on the pre release!