The only remaining client of ScrollableGrid (the old version) is Pesto,
which needs AppBar integration.

SliverFill fills the remaining space in the viewport with each box
child. We'll use this sliver as a building block for pageable lists.

These functions were previously hidden inside other functions but they
are useful enough to expose directly. Specifically, I plan to use these
when implementing Scrollable2.ensureVisible.

* Print the error message multiple times to avoid overlap with status bar and add paragraph to mock_canvas

This patch adds grid supports to slivers and introduces a ScrollGrid
convenience class for making the common types of scrollable grids.

This patch also deploys ScrollGrid in an example in the Flutter Gallery.

Our previous behavior of hard-coding the unselected label color to 70% of the
selected color was too restrictive.

Fixes #7738

Now that ScrollView supports itemExtent, we can use it in a few more places.

Add SliverList

A SliverList is a linear layout of box children in a viewport that all
have a common, fixed extent along the scroll axis. The layout is similar
to a SliverBlock but more efficient.

Fixes https://github.com/flutter/flutter/issues/4620

Fixes for: https://github.com/flutter/flutter/issues/7570 https://github.com/flutter/flutter/issues/7231 https://github.com/flutter/flutter/issues/2841 and others

ValueKey and ObjectKey shouldn't be == with subclasses.
Fixes https://github.com/flutter/flutter/issues/3107

Clean up toString for the keys a bit.

Add a test for keys.

The needsLayout getter is prone to misuse. See discussion on #3083.

This structure makes it easier to customize the viewport used in a Scrollable.
Now Scrollable2 doesn't commit to using slivers.

Uses SliverPadding to implementing paddding in ScrollView. Also, deploy
ScrollView in more places now that it implements padding.

Finally, remove loader_app.dart because it is not referenced.

We now report an accurate estimate when the lastIndex is the final index
in the list.

Also, we now project forward from the lastIndex using the extent of the
reified children instead of trying to project forward from index 0.

This new algorithm leads to estimation error as you reach the end of a
list, avoiding some ballistic overscrolls.

Rumours have it that this is faster now.

Fixes https://github.com/flutter/flutter/issues/5154

When we start a ballisitic scroll activity to correct an out-of-range scroll
offset, make sure we start with a velocity that is headed in the right
direction.

We converted all code to use method generics in #7235, but
we shouldn't have touched the `http` folder since this code
is pulled from upstream.

When you put a box in a Viewport2, it might paint outside its allocated bounds.
This patch teaches Viewport2 to clip when that happens.

This patch introduces ScrollView, which is a convenience widget for using a
SliverBlock. This patch also switches a number of tests from Block to
ScrollView. Once we support more features of block (e.g., padding and
shrinkwrapping), we'll be able to move over more clients.

I plan to use this to implement similar logic in SliverPadding.

To make this easier to test I extended the paints matcher to accept a
function that takes a canvas. While I was at it I also made it accept
a Finder, it'll go and find the render object for you.

Also added support for paints..path and fixed some grammar in the
error messages.

Also improved the docs for debugPaint*.

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.