Add BoxConstraints.isNormalized feature.
Use this feature in asserts in all the intrinsic dimension methods, in
various relevant BoxConstraints methods, and in layout().
Wrap the _DebugSize logic in BoxConstraints.constrain() in an assert
block to avoid a branch in release mode.
Remove the logic in BoxConstraints.isSatisfiedBy() that dealt with
non-normalized values.
Add BoxConstraints.normalize().
Make RenderCustomOneChildLayoutBox.performLayout() only set
parentUsesSize on the child if the constraints aren't tight.
