* re-enable lint unnecessary_const

* remove trailling whitespaces

* remove unnecessary const (after merge)

This reverts commit cc1cf13e.

The new matcher package deprecates isInstanceOf which seems eggregious.

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.

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)

This should reduce the number of flakes without actually increasing
the timeout, so we'll still find out quickly if a test is hanging.

The numbers here might need tweaking. Maybe the default two seconds is
too short for CI bots.

Now that Dart 1 is turned off, reapplying my change to turn on the prefer_generic_function_type_aliases analysis option, and fix all the typedefs to Dart 2 preferred syntax.

Also eliminated the unused analysis_options_repo.yaml file and turned on public_member_api_docs in analysys_options.yaml.

No logic changes, just changing the typedef syntax for all typedefs, and updating analysis options.

Revert "Update typedef syntax to use Function notation and turn on lint for old notation. (#18035)" (#18041)

This reverts commit 32586020.

Fixes #18028

Just changes typedef declarations, no logic changes.

There's no reason for them to read/write.

1. Make goldenFileComparator getter return `null` if it's set to the
   uninitialized comparator, which matches the behavior of the setter
   (it sets it to the uninitialized comparator if the caller specifies
   `null`).
2. Make the uninitialized comparator return trivial success (and print
   a message) when asked to compare as opposed to throwing. This ensures
   that the comparator will play nicely with live widget bindings
3. Augment documentation
4. Add assert that test doesn't modify the value of `autoUpdateGoldenFiles`

It allows callers to account for things like shadows that paint
outside the bounds of the widget.

https://github.com/flutter/flutter/issues/16859

* Add a `matchesGoldenFile()` async matcher that will match
  a finder's widget's rasterized image against a golden file.
* Add support for pluggable image comparison backends
* Add a default backend that does simplistic PNG byte
  comparison on locally stored golden files.
* Add support for `flutter test --update-goldens`, which will
  treat the rasterized image bytes produced during the test
  as the new golden bytes and update the golden file accordingly

Still TODO:

* Add support for the `flutter_test_config.dart` test config hook
* Utilize `flutter_test_config.dart` in `packages/flutter/test`
  to install a backend that retrieves golden files from a dedicated
  `flutter/goldens` repo

https://github.com/flutter/flutter/issues/16859

Document missing value types supported by `within`. Add missing tests. Simplify _sizeDistance implementation.

libtxt is now the default text renderer

For backward compatibility we keep supporting specifying the shape as a
combination of MaterialType and borderRadius, and we just use that as a
default when shapeBorder is null.

To cleanup the implementation if shapeBorder was not specified we just
translate the specified shape to a shapeBorder internally.
I benchmarked paint, layout and hit testing, with the specialized shape
clippers vs. the equivalent path clippers and did not see any
significant performance difference.

For testing, I extended the clippers/physicalShape matchers to match either the
specialized shape or the equivalent shape.

* Remaining code changes for Dart 2.0 SDK roll

* make time picker accessible

* use new CustomPaint a11y API

* flutter_localizations tests; use bigger distance delta

* fix am/pm control; selected values

* fix translations; remove @mustCallSuper in describeSemanticsConfiguration

* exclude AM/PM announcement from iOS as on iOS the label is read back automatically

I got tired of drive-by spelling fixes, so I figured I'd just take care of them all at once.

This only corrects errors in the dartdocs, not regular comments, and I skipped any sample code in the dartdocs. It doesn't touch any identifiers in the dartdocs either. No code changes, just comments.

* Refactor SemanticsData and SemanticsNode to use Diagnosticable.

* Switch toStringDeep to take named parameters.

* Add hasAGoodToStringDeep and equalsIgnoringHashCodes methods.

Methods simplify testing of toStringDeep calls and other cases where
methods return strings containing hash codes.

Also, I had a question about flutter_test matchers and our style guide
says that when I have a question I should update the docs so I did
that and then got a bit carried away.

Fixes #7734