[tool] Improve using project files in build targets (#166211)
The Flutter tool's build system executes targets that take inputs and
produces outputs. Previously, targets would hardcode paths if they
needed to use Flutter project files as inputs/outputs. For example:
```dart
class FooTarget extends Target {
@override
List<Source> get inputs => <Source>[
Source.pattern('{PROJECT_DIR}/foo/bar'),
];
}
```
This is problematic as the
[`FlutterProject`](05b5e79105/packages/flutter_tools/lib/src/project.dart (L89))
is the source of truth for where a Flutter project's files are located:
1. If we change a project file's location, we need to update
`FlutterProject` as well as any hardcoded target inputs/outputs.
2. Project files' location can be dynamic. For example, a Flutter app
puts iOS files in the `ios/` directory, but a Flutter module puts iOS
files in the `.ios/` directory. Targets need to duplicate
`FlutterProject`'s logic to determine the project file's location.
As a result, hardcoding project file paths in targets can be
error-prone.
This introduces a new `Source` factory that lets you use the
`FlutterProject` to create the source:
```dart
class FooTarget extends Target {
@override
List<Source> get inputs => <Source>[
Source.fromProject((FlutterProject project) => project.fooFile),
];
}
```
Part of https://github.com/flutter/flutter/issues/163874
Next pull request: https://github.com/flutter/flutter/pull/165916
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- [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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[Tree Hygiene]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
[test-exempt]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
[Flutter Style Guide]:
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[CLA]: https://cla.developers.google.com/
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[breaking change policy]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes
[Discord]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md
[Data Driven Fixes]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md
a48a1f07b8