Add find.descendant API to support find descendants of an element by passing the current element finder and the finder of descendant. (#7944)

* Add find.descendant API to support find descendants of an element by passing the current element and the finder of descendant.

* Add find.descendant API to support find descendants of an element by passing the current element finder and the finder of descendant.

* Add find.descendant API to support find descendants of an element by passing the current element finder and the finder of descendant.

* Add find.descendant API to support find descendants of an element by passing the current element finder and the finder of descendant.

* Add find.descendant API to support find descendants of an element by passing the current element finder and the finder of descendant.

* Add find.descendant API to support find descendants of an element by passing the current element finder and the finder of descendant.

* Add find.descendant API to support find descendants of an element by passing the current element finder and the finder of descendant.

* Add find.descendant API to support find descendants of an element by passing the current element finder and the finder of descendant.

* Add find.descendant API to support find descendants of an element by passing the current element finder and the finder of descendant.

* Add find.descendant API to support find descendants of an element by passing the current element finder and the finder of descendant.

* Add find.descendant API to support find descendants of an element by passing the current element finder and the finder of descendant.

* Support thinning iOS frameworks to supported architectures

When building against frameworks that are distributed as
multi-architecture fat binaries, we want to strip the frameworks we
distribute down to only the architectures specified in $ARCHS.

This patch adds:
* The ability to specify commands to xcode_backend.sh (if none is
  specified, run BuildApp for backward compatibility).
* A 'thin' command that invokes lipo to thin down the distributed as
  described above.

* Add framework thinning step to iOS build

Invokes xcode_backend.sh thin on the build application.

* Limit architectures to arm64 in Xcode template

Flutter does not yet support armv7 iOS devices. Limit the $ARCHS build
variable to arm64 until then.

* has all the cherry-picks fixing known issues merged.

Also, move the creation of the ScrollPosition to ScrollController.

Finally, remove TestScrollable in favor of CustomScrollView, which is the
production version of this widget.

If you pass a ScrollController to a Scrollable2, you can use the controller to
read and write the scroll offset without having to find the Scrollable2State
object.

Reason: https://github.com/flutter/flutter/issues/4273

In particular, we now use ListView in the about dialog.

Required to run tests on Windows bots.

These arguments are actually required because we assert that they're not null.

These widget don't have any clients anymore. Consider using GridView instead.

* Small Flutter strong mode cleanup fixes.

These are cases where strong mode down cast composite errors
generally indicated cases that would performance or correctness
issues if Flutter code was run in a strong mode VM.

* Fix Command API so that it is always in terms of Map<String,String>.

* Fix typedef

After this patch, the old grid code is not used in the framework.

This patch uses sliver-based two more gallery demos, the stocks example,
in the date picker, and in markdown.

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.

We now create a GridLayout object that memoizes the layout information
for a given SliverConstraints. This approach is both more efficient
(because the delegate no longer needs to re-solve its layout for each
child) and allows for more code sharing between delegates.

The SliverGridRegularTileLayout will also be useful for the date
picker.

Also, document the grid delegates.

This patch moves the resonsibility for wrapping repaint boundaries
around children to SliverChildDelegate, which means delegates can choose
whether or no to use repaint boundaries.

Also introduce SliverChildBuilderDelegate to make it easier to use the
builder pattern with sliver lists.

This functionality will be used by date picker, which wants to use a
SliverGrid but doesn't need a repaint boundary around every day in a
month grid.

Someone on stack overflow was mutating the list of recipes and getting
confused about why the UI didn't redraw. Making kPestoRecipes a constant
might help avoid that confusion.

* allow mulitple observers for Navigator and MaterialApp

* add test for the navigator observers

* fix style

* add test for adding/removing a navigator observer

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

This quantity is actually the layoutOffset of the child, not its scroll offset.

This widget has no clients. Please use GridView.extent instead.

If ensureDirectoryExists fails -- e.g. because a file file of the same
name as the directory to be created exists, ensure that we exit cleanly
with a useful error message.

Allows for better testing of functions that throw ToolExit to bail out.

This patch passes down the viewportMainAxisExtent so that RenderSliverPage can
pick it up and size its children appropriately.

This prepares us for a CustomScrollView that takes slivers.

This patch makes Scrollable2.ensureVisible with SingleChildScrollView. A future
patch will extend the implementation to work with slivers. (Although the patch
does include some of the infrastructure for that part of the implementation as
well.)