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.
