[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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Documentation for the AutoRoller is here:
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* Fix "the a" typos

* Rephrase global key API doc description

* Introduce image_list performance benchmark that runs on jit(debug) build.

We want to track performance of debug build because ongoing work on reducing the cost of spanwing new isolate will impact jit(debug) more than it will aot(release) configurations.

* Newlines and spaces

An upcoming change in the Dart SDK changes `Socket` from
implementing `Stream<List<int>>` to implementing `Stream<Uint8List>`.
This forwards-compatible change in flutter_tools prepares for that
Dart SDK change.

https://github.com/dart-lang/sdk/issues/36900

https://github.com/flutter/engine/compare/7d3e722349e7...3c51a7bfff77

git log 7d3e722349e7923115f0b8800022a08c8dd795b3..3c51a7bfff776e9e3011384f4ff09c13a697c6b6 --no-merges --oneline
3c51a7bff Roll src/third_party/skia 11eb847a2080..857c9f955edb (2 commits) (flutter/engine#9676)
7f828dd3e Raster now returns an enum rather than boolean (flutter/engine#9661)
11b6afe00  Roll src/third_party/dart 67ab3be10d...b5aeaa6796 (flutter/engine#9675)
3c4dbe223 Revert " Roll src/third_party/dart 67ab3be10d...43891316ca (#9670)" (flutter/engine#9673)
5e596f2ac  Roll src/third_party/dart 67ab3be10d...43891316ca (flutter/engine#9670)
46a2239b4 Roll src/third_party/skia 5b52c52141ac..11eb847a2080 (6 commits) (flutter/engine#9671)
b84f89b44 Allow embedders to add callbacks for responses to platform messages from the framework. (flutter/engine#9655)
8dac2e979 Begin separating macOS engine from view controller (flutter/engine#9654)
d3616c7a3 Roll src/third_party/skia 0e0113dcbd9a..5b52c52141ac (8 commits) (flutter/engine#9665)
cea2c3617 Mutators Stack refactoring (flutter/engine#9663)
6a8782fe0 Roll src/third_party/skia 93eeff578b08..0e0113dcbd9a (9 commits) (flutter/engine#9662)
791143f17 ExternalViewEmbedder can CancelFrame after pre-roll (flutter/engine#9660)
d637f291a External view embedder can tell if embedded views have mutated (flutter/engine#9653)
ceee3d733 Roll src/third_party/skia 38ae3f42fec1..93eeff578b08 (1 commits) (flutter/engine#9659)
d75729006 Roll src/third_party/skia febc162c7898..38ae3f42fec1 (3 commits) (flutter/engine#9658)
58133abce Roll src/third_party/skia 3de5c6388142..febc162c7898 (2 commits) (flutter/engine#9657)
29342dd30 Roll src/third_party/skia 1e2cb444e0c1..3de5c6388142 (8 commits) (flutter/engine#9656)
b5473560d Pipeline allows continuations that can produce to front (flutter/engine#9652)
8306ee68c Move the mutators stack handling to preroll (flutter/engine#9651)
511b9f23a Roll src/third_party/skia 215ff3325230..1e2cb444e0c1 (12 commits) (flutter/engine#9650)

The AutoRoll server is located here: https://autoroll.skia.org/r/flutter-engine-flutter-autoroll

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+/master/autoroll/README.md

If the roll is causing failures, please contact the current sheriff (jsimmons@google.com), and stop
the roller if necessary.

This is necessary for those who wish to comply with static analysis like
`strict-raw-types: true`, which includes generated code within their package.