This reverts commit 71e9bd6f.

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

* Upgrade everything except matcher.
* Roll matcher (and test)
* Adjust tests that depend on flutter:test directly to depend on a shim
* Require use of package:test shim and remove other references to package:test

Also, fixes "flutter run" for golden tests, and some docs.

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`

This will allow external tools that wrap our test harness to share the
code that generates the test bootstrap.

This change exposed an issue whereby the LocalGoldenFileComparator
was being too strict in its URI handling, so this changes relaxes
that constraint as well (and adds associated tests).

* Exclude flutter_goldens package from dartdoc because it's for internal
  use only
* Document why flutter_tools doesn' tneed to be excluded from the list of
  packages to document
* Performance optimization in the flutter comparator, and associated
  test updates.

In order to avoid checking binaries into flutter/flutter,
this comparator can be used to retrieve golden files from
a sibling flutter/goldens repository.

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

This enables support for a `flutter_test_config.dart` configuration file,
which will be discovered and handed the responsibility of running the
test file (thus allowing it to run pre-test setup on a project level).

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