sort part directives (#154456)

A pending change to the `directives_ordering` lint will enforce part
sorting.

See: https://github.com/dart-lang/linter/issues/4945

And `flutter-analyze` failure:


https://logs.chromium.org/logs/dart/buildbucket/cr-buildbucket/8738142237425031297/+/u/analyze_flutter_flutter/stdout

Related to change: https://dart-review.googlesource.com/c/sdk/+/383023

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there for submitting PRs.
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responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

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packages/flutter_tools/lib/src/reporting/reporting.dart

@@ -22,7 +22,7 @@ import '../globals.dart' as globals;
 import '../version.dart';
 import 'first_run.dart';
 
+part 'custom_dimensions.dart';
 part 'disabled_usage.dart';
 part 'events.dart';
 part 'usage.dart';
-part 'custom_dimensions.dart';