## Description
Also update the existing dart files with missing licenses.
Without the fix, we'll emit the following error message
```
License headers cannot be found at the beginning of the following files.
/usr/local/google/home/liyuqian/flutter/flutter/packages/flutter/lib/src/animation/tween_sequence.dart
/usr/local/google/home/liyuqian/flutter/flutter/packages/flutter/test/material/raw_material_button_test.dart
/usr/local/google/home/liyuqian/flutter/flutter/packages/flutter/test/widgets/async_lifecycle_test.dart
/usr/local/google/home/liyuqian/flutter/flutter/packages/flutter/test/widgets/sliver_constraints_test.dart
/usr/local/google/home/liyuqian/flutter/flutter/packages/flutter/test/widgets/app_test.dart
/usr/local/google/home/liyuqian/flutter/flutter/packages/flutter/test/widgets/test_border.dart
/usr/local/google/home/liyuqian/flutter/flutter/packages/flutter/test/widgets/physical_model_test.dart
/usr/local/google/home/liyuqian/flutter/flutter/packages/flutter/test/widgets/inherited_model.dart
/usr/local/google/home/liyuqian/flutter/flutter/packages/flutter_tools/lib/src/base/user_messages.dart
/usr/local/google/home/liyuqian/flutter/flutter/packages/flutter_tools/test/src/pubspec_schema.dart
/usr/local/google/home/liyuqian/flutter/flutter/packages/flutter_tools/test/ios/simulators_test.dart
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
```
## Related Issues
Fixes https://github.com/flutter/flutter/issues/28368
## Checklist
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- [x] I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
- [x] My PR includes tests for *all* changed/updated/fixed behaviors (See [Test Coverage]).
- [x] All existing and new tests are passing.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] The analyzer (`flutter analyze --flutter-repo`) does not report any problems on my PR.
- [x] I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I am willing to follow-up on review comments in a timely manner.
## Breaking Change
Does your PR require Flutter developers to manually update their apps to accommodate your change?
- [ ] Yes, this is a breaking change (Please read [Handling breaking changes]).
- [x] No, this is *not* a breaking change.
<!-- Links -->
[issue database]: https://github.com/flutter/flutter/issues
[Contributor Guide]: https://github.com/flutter/flutter/wiki/Tree-hygiene#overview
[Test Coverage]: https://github.com/flutter/flutter/wiki/Test-coverage-for-package%3Aflutter
[Flutter Style Guide]: https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
[Features we expect every widget to implement]: https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[Handling breaking changes]: https://github.com/flutter/flutter/wiki/Tree-hygiene#handling-breaking-changes
* Change assert_bundle_package_font_test to memory file system
This is to work towards being able to run the tests without `-j1` (#21113). These tests were using the real filesystem and setting/relying on fs.currentDirectory. There was a comment about this being because the memory provider didnt' support POSIX and Windows, however that seems to have changed since (and many other asset tests already do something similar to this).
* Trim trailing whitespace
* Add a workaround for Windows path slash directions
Strictly this is correct, but the real FS can tolerate either path. The in-memory file system is more strict (see https://github.com/google/file.dart/issues/112).
* Extract a helper for writing schema files in tests
* Missed file when saving!
* Remove redundant comment
* Rename writeBasicSchema -> writeEmptySchema
* Use the file we already have to write contents
* Make comments more descriptive
* Remove another dupe of writeSchema to use the shared one
* Rename schema -> pubspec_schema
* Trim whitespace
