add a readme for the dart fix tests (#73761)

add a readme for the dart fix tests

packages/flutter/test_fixes/README.md

+## Directory contents
+
+The Dart files and golden master `.expect` files in this directory are used to
+test the `dart fix` framework refactorings used by the Flutter framework.
+
+<!-- TODO(devoncarew): Add link to public user guide when available. -->
+
+See the flutter/packages/flutter/lib/fix_data.yaml file for the current package:flutter
+data driven fixes.
+
+## When making structural changes to this directory
+
+Note that the tests in this directory are also invoked from external repositories.
+Specifically, the CI system for the dart-lang/sdk repo runs these tests in order to
+ensure that changes to the dart fix file format do not break Flutter.
+
+See [tools/bots/flutter/analyze_flutter_flutter.sh](https://github.com/dart-lang/sdk/blob/master/tools/bots/flutter/analyze_flutter_flutter.sh)
+for where the tests are invoked.
+
+When possible, please coordinate changes to this directory that might affect the
+`analyze_flutter_flutter.sh` script.