Initializes fields for subclasses.
The paintOffset and mainAxis arguments must not be null.
This constructor uses positional arguments rather than named arguments to work around limitations of mixins.
Source
RenderViewportBase(
Offset paintOffset,
Axis mainAxis,
ViewportAnchor anchor
) : _paintOffset = paintOffset,
_mainAxis = mainAxis,
_anchor = anchor {
assert(paintOffset != null);
assert(mainAxis != null);
assert(_offsetIsSane(_paintOffset, mainAxis));
}