Remaining known issues are #10831 and #10830.

Fixes https://github.com/flutter/flutter/issues/9342

backgroundColor -> color
backgroundImage -> image
BackgroundImage -> DecorationImage

* more widget const constructors

* prefer const constructors

* address review comments

* make @immutable const

* fix build

Rename State.config to State.widget
Rename State.didUpdateConfig to State.didUpdateWidget
Renamed all State subclasses' local variables named config to something else

Aggressively apply the const lint.

* add @required when there's an assert not null

* address review comments

Fixes #8235

This machinery is useful for arbitrary boxes (e.g., with FittedBox).

Fixes #6463

The new name follows the pattern of the name suggesting which layout
protocol the parent expects the child to speak.

Fixes #8664

This happens when, for instance, you go back then forward quickly.

Fixes #7734

Rename all the "2" classes related to scrolling to remove the "2". Now that the
old scrolling code is gone, we don't need to use the suffix.

Clients should use ListView instead.

Move the back button and drawer opening logic into the app bar.

Move the tap-status-bar-to-scroll-to-top logic to using
ScrollControllers. Provide a PrimaryScrollController and a `primary`
flag on scroll views.

Make it possible to track when a route becomes or stops being poppable.

This patch converts the Shrine home page to using a sliver-based grid.
This required using a CustomScrollView to mix the block at the top with
the grid below.

Also, use CustomScrollView in Shrine and fix a bug with one-line grids
not painting properly due to their reporiting zero paintExtent.

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 #6960
Fixes #5169

These futures complete when the route is popped off the navigator. This
generalizes and simplifies a mechanism already in place for dialogs and
menus.

Fixes #5283

Fixes #4713.

Fixes #3208.

* Removed unnecessary Shrine hero logic

* Shrine tile height now hardcoded to match actual card size.
* Animation demo now scales with screen size.

BUG=https://github.com/flutter/flutter/issues/5002
BUG=https://github.com/flutter/flutter/issues/5003