* Add doc example to addLocalHistoryEntry in LocalHistoryRoute (#9392)

The new matcher package deprecates isInstanceOf which seems eggregious.

Fix test.

This makes the buildbot green again.

* Add docs to Opacity recommending AnimatedOpacity for opacity animations (#15464)

Reported for the hello_world compile benchmark in devicelab for now.

This adds an HSLColor class which uses a perceptual color space based upon human perception of colored light (as opposed to HSV, which is based on pigment colors).

You can see the difference in the color spaces here: https://en.wikipedia.org/wiki/HSL_and_HSV

I also added a "within" matcher for both HSLColor and HSVColor that will check if the (floating point) color components are within a certain error.

And tests.

* Roll the engine.

This engine includes a VM that defaults to sync-async.

flutter/engine@c8eeee4 Roll the Dart VM.

Seems like VS Code may have started using lowercase extension folders recently (our identifier is `Dart-Code.flutter` but now the folder is named `dart-code.flutter`), so this makes the check not sensitive to casing.

Also reuse extension identifier for download link

... and make sure they fail if they can't report the size.

This reverts commit 77508722.

We have many hidden children because of iOS accessibility (see also #18410).

For example, in the tile screen of complex_layout app, there could be up to 20 children being painted without this PR. With this PR, only 12 get painted.

This should improve the performance of FL-53 by 40%.

Add flutter tool support for creating app-specific VM snapshots.

Fixes #10826.

Also in this PR: If you focus a text field, scroll it out of view and then start typing it will scroll back into view.

This reverts commit e593dc70.

"flutter update-packages --force-upgrade"

(second attempt)

When setState() calls occur asynchronously, it's possible that the
Futures on which they're waiting don't complete until the widget is
offscreen. To avoid this, we check the widget is mounted before calling
setState().

* Fix readme link to plugin edit instructions

* Fix flutter tools link

fuchsia_tester.dart still assumes Dart 1. Previously, it ran tests directly
from source, flutter_platform.dart automatically runs a kernel compile when
operating in Dart 2 mode, but this assumes a functional Dart SDK is available
in the artifacts directly, and fuchsia_tester.dart mocks out the artifacts
directory with an empty temp dir.

Remaining work is:
1. Get the frontend server building as a dependency on Fuchsia.
2. Patch fuchsia_tester.dart to use a valid Dart SDK and frontend server.

This also reverts migration to Dart 2 typedef syntax.

This reverts commit 6c56bb24. (#18362)
This reverts commit 3daebd05. (#18316)