- Fixed extentInside calculation in ScrollMetrics
- Added asserts to extentInside getter, as well as ScrollPosition.applyContentDimensions to enforce minScrollExtent <= maxScrollExtent
- Added padding to ScrollbarPainter, updated implementation. Took care of some edge cases.
- Changed some scroll bar constants on Cupertino side.

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

* Adjust the defaults behaviour of scroll views.

Now, primary scroll views scroll by default. Others only scroll if necessary.

* apply suggested changes

This patch reworks some of the guts of scrolling to make it easier to
implement nested scrolling effects. The actually nested scrolling effect
will be included in a later patch.