Change focus example to be more canonical (and correct), listening to the focus node for changes. (#35913)

This changes the example for FocusNode to be more correct, listening to the focus node for changes, instead of assuming that it is the only one doing the changing.

This adds an "anchor button" to each of the samples so that the user can link to individual samples instead of having to link to just the page. Clicking on the anchor button jumps to the anchor, as well as copying the anchor URL to the clipboard.

There is some oddness in the implementation: because dartdoc uses a <base> tag, the href for the link can't just be "#id", it has to calculate the URL from the current window href. I do that in the onmouseenter and onclick because onload doesn't get triggered for <a> tags (and onmouseenter doesn't get triggered for mobile platforms), but I still want the href to be updated before someone right-clicks it to copy the URL.

Show and hide the text selection menu at the correct time with various gestures in the text field.

* Remove extra classes.

Now that https://github.com/dart-lang/sdk/issues/31543 is fixed, we
can remove extra classes.

* Add snapConfiguration to parameters for _RenderSliverFloatingPersistentHeaderForWidgets and _RenderSliverFloatingPinnedPersistentHeaderForWidgets constructors.

Fixed build of example code to use new binary messenger API.

* Add flag to use root navigator for showModalBottomSheet

* Add documentation

* Add tests

* Address feedback and fix analyzer issues

* Address feedback

* Update comments

* Fixed assets embedding

* ++

* Completed snippet audit

* Analyzer

This introduces a script that implements the conventions described in https://github.com/flutter/tests/pull/1.

* Add Colors.white38

* Update ThemeData.disabledColor and ButtonThemeData.disabledColor to Colors.white38

* Update pre-existing tests to expect Colors.white38 instead of Colors.white30

* Update API documentation to reflect these changes

When I added reverseDuration to animation controllers in #32730, I also added it to the implicit animations that Flutter has. However, as @efortuna pointed out to me, it doesn't actually do anything there, since all of the intrinsic animations run forwards, not in the reverse direction, and there's no way to reverse them.

So, this PR removes the reverseDuration argument from the implicit animations to avoid confusion.

Fixes #35769

* add sample code for AnimatedContainer

* use stateful_widget_scaffold snippet template

* add sample explanation

* add setState() to example assumptions for analyzer

* update description

* use snippet instead of sample

* change sample to match assets-for-api-docs

* remove constant

* update AnimatedContainer sample description

add indication the example is depicted by the video and animates when
tapped

* fix example formatting

* make sample conform to 80 col limit

* add "implemented below" to illustration description

* formatting for readability

Added support for a themeMode property to the MaterialApp to control
how the light or dark theme is selected.

[Material] Text scale and wide label fixes for Slider and Range Slider value indicator shape (#35496)

Various bug fixes for Slider and Range Slider value indicator around accessibility and wide labels.

Rendering errors with root causes in the widget layer should have a reference to the widget (#32511)

* Initial work

* ++

* Updated tests

* Moved TestObserver out for access across tests.

* ++

* Added hero tests

* Review feedback

* simplified preceding route overlay

* Review feedback

* trailing doc slash

* Add missing [applicationIcon] property to LicensePage widget

For https://github.com/flutter/flutter/issues/31086.

Also test if our mac8 with iPhone XS is working
(https://github.com/flutter/flutter/issues/34590).

https://github.com/flutter/engine/compare/3c51a7bfff77...e695a516f148

git log 3c51a7bfff776e9e3011384f4ff09c13a697c6b6..e695a516f148cef9c7850a6e00151f00bfadf0d4 --no-merges --oneline
e695a516f Add FLEDartProject for macOS embedding (flutter/engine#9672)
44f2f07ff Roll src/third_party/skia e192c4ce5a33..829b6a0ff49f (1 commits) (flutter/engine#9705)
d5f15d1cd Roll src/third_party/skia de6758683abd..e192c4ce5a33 (2 commits) (flutter/engine#9703)
e46532986 Roll src/third_party/skia 2f62edf1ac5d..de6758683abd (1 commits) (flutter/engine#9700)
05966cd14 Roll src/third_party/skia 1d105080c423..2f62edf1ac5d (1 commits) (flutter/engine#9699)
b132baa44 Ensure that platform messages without response handles can be dispatched. (flutter/engine#9698)
50d622857 fix Picture.toImage return type check and api conform test. (flutter/engine#9685)
001bb541c Roll src/third_party/skia 8063f6cca025..1d105080c423 (3 commits) (flutter/engine#9692)
db4d6836a Roll src/third_party/skia 857c9f955edb..8063f6cca025 (3 commits) (flutter/engine#9679)

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