Instead of using mixins and subclasses, we now use a `ScrollPhysics` delegate
to customize the scrolling phyics for Android and iOS. This change will make it
easier to customize the scroll position for paging.

Also emits the version information in flutter doctor output.

ios-deploy > 1.9.0 is required to deploy to iOS devices.

* Return null from VM.mainView if no view exists
* Retry in connectToServiceProtocol if a view is not yet available
* Do not explicitly call exit from the benchmarks - it will not cleanly shut down the engine

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.

This patch removes unnecessary uses of scrollableKey from tests.
Scrollable2 will likely use a different mechanism for identifying
itself, so we want to focus on the uses of scrollableKey that will need
to be addressed in the new implementation.

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.

This avoids Xcode complaining at us about unused xcconfig files (for the
time being).

* Update gradle example to support x86 in debug mode.

Changed the Flutter Gradle plugin a bit to better fit in with the
Android build.

Fixes #6136
Fixes #6864
Fixes #7539

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

This update includes the fix for the search box.

It was causing the flakiness because it was too short. The default timeout is better.

These tests would fail sometimes when run locally. Since they happen
to also be the ones that are flaky on bots, maybe there's some
relationship, even though the failure mode on bots is different. In
any case, this removes a variable.

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.

The finder will only match the widget's ValueKey if both have identical
runtime types

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.

Reverts a change from https://github.com/flutter/flutter/pull/3158.
Fixes https://github.com/flutter/flutter/issues/3161