* Breaking change which sets the default DragStartBehavior to DragStartBehavior.start and changes WidgetTester.drag and WidgetTester.dragFrom by breaking the moveBy function call into multiple pieces.
This change make it easier to track the position of the scroll view without
having to worry about the position object changing out from under the
controller.
* Improved defaults for scroll view primary-ness
* Vertical scroll views default to primary:true.
* Horizontal scroll views default to primary:false.
* If a scroll view is primary and it got a non-null inherited primary
scroll controller, it introduces a primary scroll controller inherited
with a value of null for its descendants.
ScrollController now multiplexes writes to all registered positions;
reads of position continue to assert that only one position is
registered. Reads still require a single position.
Also, move the creation of the ScrollPosition to ScrollController.
Finally, remove TestScrollable in favor of CustomScrollView, which is the
production version of this widget.
If you pass a ScrollController to a Scrollable2, you can use the controller to
read and write the scroll offset without having to find the Scrollable2State
object.
