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## Description
This PR introduces platform-specific asset support in `pubspec.yaml`.
Currently, Flutter does not allow specifying which platforms an asset
should be included for.
This results in all declared assets being bundled for every target
platform, even if some are irrelevant (e.g. desktop-only or mobile-only
images).
### What this PR changes
- Adds a new optional `platforms` field under each asset in
`pubspec.yaml`.
- The field accepts a list of strings (platform identifiers, e.g.
`["android", "ios", "web", "windows", "macos", "linux"]`).
- Assets with a `platforms` restriction are only included in the bundle
when building for a matching platform.
- Invalid values (non-strings or unknown platform names) log an error.
### Example
```yaml
flutter:
assets:
- path: assets/logo.png
- path: assets/web_worker.js
platforms: [web]
- path: assets/desktop_icon.png
platforms: [windows, linux, macos]
```
#### Before
All assets (`logo.png`, `web_worker.js`, `desktop_icon.png`) are bundled
into **every build**, regardless of platform.
#### After
- `logo.png` is included on all platforms.
- `web_worker.js` is included only on web builds.
- `desktop_icon.png` is included only on desktop builds.
### Why this is useful
This significantly improves bundle size, prevents unused resources from
being shipped, and gives developers better control over asset
management.
## Issues
Fixes #65065
## Reviewer note
Would a design document be helpful for this change, or is the current
explanation sufficient?
## Pre-launch Checklist
- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
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.
<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview
[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]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[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