Add hotfix instructions to DWDS (#1876)

Author: elliette

dwds/CONTRIBUTING.md

@@ -19,8 +19,8 @@ alias flutter_tools='/YOUR_PATH/flutter/bin/dart --observe /YOUR_PATH/flutter/pa
 > - `/PATH_TO_YOUR_FLUTTER_REPO/packages/flutter_tools/bin/flutter_tools.dart`:
 >   This is the path to Flutter Tools itself
 >
-> *More details can be found at the Flutter Tools
-> [README](https://github.com/flutter/flutter/blob/master/packages/flutter_tools/README.md).*
+> _More details can be found at the Flutter Tools
+> [README](https://github.com/flutter/flutter/blob/master/packages/flutter_tools/README.md)._
 
 3. In your Flutter Tools
    [`pubspec.yaml`](https://github.com/flutter/flutter/blob/master/packages/flutter_tools/pubspec.yaml),
@@ -73,8 +73,8 @@ the example app and connect to DWDS.
 
 ### Step 1: Roll DWDS into g3
 
-> *NOTE: You must be a Googler to do this step. If you are not, please ask
-> someone for help.*
+> _NOTE: You must be a Googler to do this step. If you are not, please ask
+> someone for help._
 
 - See directions at: go/roll-dwds
 - Wait a few days after rolling into g3 before continuing to step 2. We do so to
@@ -91,21 +91,21 @@ the example app and connect to DWDS.
 - From `/dwds` run `dart run build_runner build`, this will build and update the
   version in `/dwds/lib/src/version.dart`
 - Submit a PR with those changes (example PR:
-  https://github.com/dart-lang/webdev/pull/1456). *Note: Ensure your PR doesn’t
-  have any dependency overrides.*
+  https://github.com/dart-lang/webdev/pull/1456). _Note: Ensure your PR doesn’t
+  have any dependency overrides._
 - Once the PR is submitted, pull from master and `run dart pub publish`
 - Finally, go to https://github.com/dart-lang/webdev/releases and create a new
   release, eg https://github.com/dart-lang/webdev/releases/tag/dwds-v12.0.0. You
   might need to delete some of the content of the autogenerated notes.
 
-> *Note: To have the right permissions for publishing, you need to be invited to
+> _Note: To have the right permissions for publishing, you need to be invited to
 > the tools.dart.dev. A member of the Dart team should be able to add you at
-> https://pub.dev/publishers/tools.dart.dev/admin.*
+> https://pub.dev/publishers/tools.dart.dev/admin._
 
 ## Step 3: Publish Webdev to pub
 
-> *Note: DWDS is a dependency of Webdev, which is why DWDS must be published
-> before Webdev can be published.*
+> _Note: DWDS is a dependency of Webdev, which is why DWDS must be published
+> before Webdev can be published._
 
 Follow instructions in the `webdev/webdev`
 [CONTRIBUTING](/webdev/CONTRIBUTING.md) to release Webdev.
@@ -138,3 +138,58 @@ you need to:
 > run tests for all earlier stable releases of the SDK that match the
 > constraint, which would have differences in functionality and therefore need
 > different tests.
+
+## Hotfixes
+
+Sometimes you might need to do a hotfix release of DWDS. An example of why this
+might be necessary is if you need to do a hotfix of DWDS into Flutter, but don't
+want to release a new version of DWDS with the current untested changes on the
+master branch. Instead you only want to apply a fix to the current version of
+DWDS in Flutter.
+
+### Instructions:
+
+1. Create a branch off the release that needs a hotfix:
+
+   a. In the Github UI's
+   [commit history view](https://github.com/dart-lang/webdev/commits/master),
+   find the commit that prepared the release of DWDS that you would like to
+   hotfix.
+
+   b. Click on `< >` ("Browse the repository at this point in history").
+
+   c. At the top-left, you should see the commit hash in a dropdown. Click the
+   dropdown, and type in a name for your hotfix branch (e.g.
+   `16.0.2-hotfix-release`). Then select "Create branch `16.0.2-hotfix-release`
+   from `commit_hash`".
+
+   d. From your local clone of DWDS, run `git fetch upstream`. (_Note: this
+   assumes you have already configured git to sync your fork with the `upstream`
+   repository. If you haven't, follow
+   [these instructions](https://docs.github.com/en/get-started/quickstart/fork-a-repo#configuring-git-to-sync-your-fork-with-the-upstream-repository).)_
+
+   e. Search for the branch that you just created, e.g.
+   `git branch -a | grep 16.0.2-hotfix-release` f. Track that branch with
+   `git checkout --track branch_name` (e.g.
+   `remotes/upstream/16.0.2-hotfix-release`)
+
+1. Update the CI tests so that the branch tests against the appropriate Dart
+   SDKs:
+
+   a. Make the appropriate changes to DWDS' `mono_pkg.yaml` then run
+   `mono_repo generate`. Submit this change to the branch you created in step
+   #3, **not** `master`.
+
+1. Make the fix:
+
+   a. You can now make the change you would like to hotfix. From the Github UI,
+   open a PR to merge your change into the branch you created in step #3,
+   **not** `master`. See https://github.com/dart-lang/webdev/pull/1867 as an
+   example.
+
+1. Once it's merged, you can follow the instructions to
+   [publish DWDS to pub](#step-2-publish-dwds-to-pub), except instead of pulling
+   from `master`, pull from the branch your created in step #3.
+
+1. If necessary, open a cherry-pick request in Flutter to update the version.
+   See https://github.com/flutter/flutter/issues/118122 for an example.