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`
* 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
* Add hasAGoodToStringDeep and equalsIgnoringHashCodes methods.
Methods simplify testing of toStringDeep calls and other cases where
methods return strings containing hash codes.
Also, make hasOneLineDescription more discerning.
Also, add a test for hasOneLineDescription.
Also, add a test for GravitySimulation, to test the toString.
```dart
expect(x, moreOrLessEquals(0.0));
```
...for those cases where `x` might be 1e-11 or whatever.
Also, be more resilient when dumping the tree from inside the test framework.
Also, add an assert that helped me debug something the other day.
