This widget is a replacement for ScrollableViewport that uses the new
Scrollable2 machinery. The widget is not based on Slivers but does use the new
scroll behavior classes.
This patch replaces uses of Flexible with Expanded where we're using
FlexFit.tight. We still need to think of a better name for the
FlexFit.loose variant.
Also, improve the docs for Row, Column, Flex, and RenderFlex to be more
problem-oriented and to give a complete account of the layout algorithn.
Fixes#6960Fixes#5169
FormField is now a widget that can contain any type of field. Input no
longer has special code to handle form fields. Instead, there is a
helper widget InputFormField for using an Input inside a FormField.
Fixes https://github.com/flutter/flutter/issues/6097 and based on
feedback from the same.
