Picks up accessibility fixes.

* preview of prefer_asserts_in_initializer_list lint

* fix issue

Create a CupertinoNavigationBar without automatic adding of the lead button

Extract layout logic in material app bar to a common file that can be reused for cupertino

* (Update and) Integrate Rendering README into code

* Add mising whitespace

* Review comments and other touch-ups

Fixes https://github.com/flutter/flutter/issues/10399

* move more tests to Linux hosts

* fix test

Eliminates nearly-duplicate install instructions for libimobiledevice,
ideviceinstaller.

Since ideviceinstaller depends on libimobiledevice, it's almost certain
that if libimobiledevice isn't installed, or needs updating, so does
ideviceinstaller.

This message will be emitted both when libimobiledevice requires
updating, or when it has not yet been installed.

It's also not specifically the version of Xcode that it's incompatible
with, it's the lockdownd daemon, which is actually more closely tied to
iTunes.

Check ThemeData instead of Theme.

Also, clean up the menus code a bit.

Also, make it easier to write a PopupMenuEntry that has itself many
items (for example, the way Chrome's menu has icons in a row).

Fixes https://github.com/flutter/flutter/issues/9923

The lint ensures that the prefix you use in your "import ... as"
directive conforms to lowercase_with_underscores style. However,
generated code often doesn't, since it may use preceding underscores
for collision avoidance, or directly use locale codes in the
prefixes.

Replace a misplaced StatelessWidget class with StatefulWidget class

You can't have a newline between the `]` and the `(` in an image in markdown it turns out.

Minor fixes throughout, e.g. removing trailing commas from the end of sample code expressions, changing headings to "sample code" more consistently, removing stale todos, fix typos in a private method name, minor grammar fixes, added some clarifications to CircularProgressIndicator, LinearProgressIndicator, CrossAxisAlignment, added some cross-references to various members, made it slightly clearer that layout algorithms are implementation details.

Clarified "elevation" throughout.

Added docs to InkResponse and InkWell.

Added sample code for: SliverAppBar, Card, ListTile, EdgeInsets, Row, Column, CustomScrollView, ListView, SliverFixedExtentList, and SliverGrid.

Fixes #10317.
Fixes #10316.
Fixes #10267. (sort of, see comment therein)
Fixes #9331. (sort of, see comment therein)
Fixes #9407. (sort of, see comment therein)

Relocates two tests alongside other related tests:
* moved code_signing_test.dart alongside other lib/src/ios tests
* moved terminal_test.dart alongside other lib/src/base tests

* Add more tests of toStringDeep.

These tests will help ensure toStringDeep behavior is not regressed
by refactoring toStringDeep to support structured data.

See https://github.com/flutter/flutter/pull/10259