## Description

This removes all of the comments that are of the form "so-and-so (must not be null|can ?not be null|must be non-null)" from the cases where those values are defines as non-nullable values.

This PR removes them from the animation, cupertino, foundation, gestures, semantics, and services libraries.  Each of them only had a few, so I lumped them together.

This was done by hand, since it really didn't lend itself to scripting, so it needs to be more than just spot-checked, I think. I was careful to leave any comment that referred to parameters that were nullable, but I may have missed some.

In addition to being no longer relevant after null safety has been made the default, these comments were largely fragile, in that it was easy for them to get out of date, and not be accurate anymore anyhow.

This did create a number of constructor comments which basically say "Creates a [Foo].", but I don't really know how to avoid that in a large scale change, since there's not much you can really say in a lot of cases.  I think we might consider some leniency for constructors to the "Comment must be meaningful" style guidance (which we de facto have already, since there are a bunch of these).

## Related PRs
- https://github.com/flutter/flutter/pull/134991
- https://github.com/flutter/flutter/pull/134992
- https://github.com/flutter/flutter/pull/134993
- https://github.com/flutter/flutter/pull/134994

## Tests
 - Documentation only change.

Reverts flutter/flutter#134953

Several failures on CI

New feature in upcoming Dart 3.2. See https://github.com/dart-lang/language/issues/2020. Feature is enabled by bumping the min SDK version to 3.2.

Part of https://github.com/flutter/flutter/issues/134476.

...and add a test so we remember to keep it in sync.

Fixes https://github.com/flutter/flutter/issues/132157

Also:
- minor improvements to documentation
- wrap one of our test error messages in a manner more consistent with other messages

Final part of https://github.com/flutter/flutter/issues/123346.

Relanding https://github.com/flutter/flutter/pull/128901

Part of https://github.com/flutter/flutter/issues/113402
Part of https://github.com/flutter/flutter/issues/127030

Fixes https://github.com/flutter/flutter/issues/131627

Originally this code sometimes was returning null and sometimes was failing, when stack frame is in unexpected format.

This PR updates for one of the code paths from failing to returning null.

This primarily implements DecorationImage.lerp().

It also makes some minor tweaks, the main one of which is defering to dart:ui for `clampDouble` instead of duplicating it in package:foundation.

Fixes https://github.com/flutter/flutter/issues/12452

This was first landed in https://github.com/flutter/flutter/pull/130533 and reverted in https://github.com/flutter/flutter/pull/131347.

Reverts flutter/flutter#130533

Tree breakage.

This primarily implements DecorationImage.lerp().

It also makes some minor tweaks, the main one of which is defering to dart:ui for `clampDouble` instead of duplicating it in package:foundation.

Fixes https://github.com/flutter/flutter/issues/12452

Fixes #115525.

On [AbstractNode.detach] and its two progeny [RenderNode.detach]
and [Layer.detach], the docs said both to call the inherited method
before detaching children, and to end by doing so.  The former
advice is what's enforced by an assertion in the base implementation,
so cut out the other.

The corresponding [attach] methods redundantly said twice to
call the inherited method first, so cut the redundancy.
Leave in place the version more recently added (in #76021), because
that PR shows the old version must have been easy to overlook.

Fixes https://github.com/flutter/flutter/issues/42390

- `ui_web.warmupEngine`
- `ui_web.setPluginHandler`
- `ui_web.debugEmulateFlutterTesterEnvironment`

Reverts flutter/flutter#128901

context: ([cl/542712088](https://critique.corp.google.com/542712088) , [b/288495136](https://b.corp.google.com/288495136))

Remove `dom.dart`'s JS interop definitions in favor of `package:web`.

Part of https://github.com/flutter/flutter/issues/113402
Part of https://github.com/flutter/flutter/issues/127030

## FlutterTimeline

Add a new class `FlutterTimeline` that's a drop-in replacement for `Timeline` from `dart:developer`. In addition to forwarding invocations of `startSync`, `finishSync`, `timeSync`, and `instantSync` to `dart:developer`, provides the following extra methods that make is easy to collect timings for code blocks on a frame-by-frame basis:

* `debugCollect()` - aggregates timings since the last reset, or since the app launched.
* `debugReset()` - forgets all data collected since the previous reset, or since the app launched. This allows clearing data from previous frames so timings can be attributed to the current frame.
* `now` - this was enhanced so that it works on the web by calling `window.performance.now` (in `Timeline` this is a noop in Dart web compilers).
* `collectionEnabled` - a field that controls whether `FlutterTimeline` stores timings in memory. By default this is disabled to avoid unexpected overhead (although the class is designed for minimal and predictable overhead). Specific benchmarks can enable collection to report to Skia Perf.

## Semantics benchmarks

Add `BenchMaterial3Semantics` that benchmarks the cost of semantics when constructing a screen full of Material 3 widgets from nothing. It is expected that semantics will have non-trivial cost in this case, but we should strive to keep it much lower than the rendering cost. This is the case already. This benchmark shows that the cost of semantics is <10%.

Add `BenchMaterial3ScrollSemantics` that benchmarks the cost of scrolling a previously constructed screen full of Material 3 widgets. The expectation should be that semantics will have trivial cost, since we're just shifting some widgets around. As of today, the numbers are not great, with semantics taking >50% of frame time, which is what prompted this PR in the first place. As we optimize this, we want to see this number improve.

It's time to say good bye to an old friend. 
It has outlived its usefulness.
Farewell, AbstractNode! 🫡

Use `package:web` and `dart:js_interop` instead.

Part of https://github.com/flutter/flutter/issues/127030

Follow-up to https://github.com/flutter/engine/pull/42493.

The newly updated lint will soon flag for-each in collections.

See discussion: https://github.com/dart-lang/linter/pull/4383

/cc @goderbauer

Follow-up to https://github.com/flutter/flutter/pull/126647#discussion_r1195417860.

This PR contains a series of minor changes to address issues that I happened to run into:

 - Pretty-print errors triggered when handling events that are pending because of having locked event handling. Previously these were just dumped to the console.
 - Add more documentation for `debugPaintPadding`.
 - Add documentation to `Tween` saying how to implement it.
 - Slight formatting changes in the scrollbar code to align some expressions.
 - Since we convert ScrollMetricsNotifications to ScrollNotifications in various places, provide an explicit API to do this. This will make the behaviour consistent throughout, and makes the code easier to understand. Added a test for this.
 - Clarifications to some of the BindingBase and SchedulerBinding documentation.
 - Clarified some documentation in `flutter_test`'s `Finder` class.

## Description

Adding some see also links to connect `defaultTargetPlatform` and `kIsWeb` docs.

dart:js_interop and package:js will start conflicting, since they both have an `@JS` annotation. Until we're ready to only use dart:js_interop (which will require updating the SDK constraints of every package), we should hide the `@JS` annotation from dart:js_interop. Due to shadowing, this is the behavior today, so there should be no functional change.

Unblocks https://dart-review.googlesource.com/c/sdk/+/294130/8 and prevents confusing shadowing of dart:js_interop annotations like we do today.

- [Mentioned CL that is unblocked] I listed at least one issue that this PR fixes in the description above.
- [Need test-exemption] I added new tests to check the change I am making, or this PR is [test-exempt].
- [Need to run] All existing and new tests are passing.

Bug report 2 issue template

Remove 1745 decorative breaks