Adjust pull request template (#509)

Align the flutter/packages template with recent changes to the flutter/plugins template.

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,31 +1,33 @@
-*Replace this paragraph with a description of what this PR is changing or adding, and why.*
+*Replace this paragraph with a description of what this PR is changing or adding, and why. Consider including before/after screenshots.*
 
 *List which issues are fixed by this PR. You must list at least one issue.*
 
 *If you had to change anything in the [flutter/tests] repo, include a link to the migration guide as per the [breaking change policy].*
 
 ## Pre-launch Checklist
 
-- [ ] The title of the PR starts with the name of the package surrounded by square brackets, e.g. `[shared_preferences]`
 - [ ] I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
 - [ ] I read the [Tree Hygiene] wiki page, which explains my responsibilities.
-- [ ] I read and followed the [Flutter Style Guide].
+- [ ] I read and followed the [relevant style guides] and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use `dart format`.)
+- [ ] I signed the [CLA].
+- [ ] The title of the PR starts with the name of the package surrounded by square brackets, e.g. `[shared_preferences]`
 - [ ] I listed at least one issue that this PR fixes in the description above.
-- [ ] I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt.
-- [ ] I updated `pubspec.yaml` with an appropriate new version according to the [pub versioning philosophy].
+- [ ] I updated `pubspec.yaml` with an appropriate new version according to the [pub versioning philosophy], or this PR is [exempt from version changes].
 - [ ] I updated `CHANGELOG.md` to add a description of the change.
 - [ ] I updated/added relevant documentation (doc comments with `///`).
-- [ ] I signed the [CLA].
+- [ ] I added new tests to check the change I am making, or this PR is [test-exempt].
 - [ ] All existing and new tests are passing.
 
-If you need help, consider asking for advice on the `#hackers-new` channel on [Discord].
+If you need help, consider asking for advice on the #hackers-new channel on [Discord].
 
 <!-- Links -->
-[Contributor Guide]: https://github.com/flutter/flutter/wiki/Tree-hygiene#overview
+[Contributor Guide]: https://github.com/flutter/packages/blob/master/CONTRIBUTING.md
 [Tree Hygiene]: https://github.com/flutter/flutter/wiki/Tree-hygiene
-[Flutter Style Guide]: https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
+[relevant style guides]: https://github.com/flutter/packages/blob/master/CONTRIBUTING.md#style
 [CLA]: https://cla.developers.google.com/
 [flutter/tests]: https://github.com/flutter/tests
 [breaking change policy]: https://github.com/flutter/flutter/wiki/Tree-hygiene#handling-breaking-changes
 [Discord]: https://github.com/flutter/flutter/wiki/Chat
 [pub versioning philosophy]: https://dart.dev/tools/pub/versioning
+[exempt from version changes]: https://github.com/flutter/flutter/wiki/Contributing-to-Plugins-and-Packages#version-and-changelog-updates
+[test-exempt]: https://github.com/flutter/flutter/wiki/Tree-hygiene#tests