collapse method
Auto margins already have a "value" of zero so can be considered collapsed.
Implementation
Margins collapse() => Margins(
left: left?.unit == Unit.auto ? left : Margin(0, Unit.px),
right: right?.unit == Unit.auto ? right : Margin(0, Unit.px),
top: top?.unit == Unit.auto ? top : Margin(0, Unit.px),
bottom: bottom?.unit == Unit.auto ? bottom : Margin(0, Unit.px),
);