check: pin flutter version

* Introduce a git submodule to pin the version of flutter SDK under
  `vendor/flutter`.
* Use direnv to add `vendor/flutter/bin` to the current PATH so that
  subsequent calls to `flutter` on the shell uses the our pinned
  version of flutter.
* Update instructions to use pinned version of flutter.
* Pin to 18340ea16c of flutter which matches the current lowerbound
version in pubsec.yaml (3.21.0-12.0.pre.26).

NOTE: Users can still choose to opt-out and use their own version of
flutter. This just provides a recommended and tested version on our
CI.

.envrc

@@ -0,0 +1,5 @@
+# This file is used by direnv to setup the environment when entering to
+# use vendored flutter SDK.
+
+# Comment out the next line if you want to use your system flutter.
+PATH_add vendor/flutter/bin

.github/workflows/ci.yml

@@ -6,17 +6,26 @@ jobs:
   check:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v3
+    - name: Checkout repository
+      uses: actions/checkout@v3
 
-    - name: Clone Flutter SDK
+    - name: Checkout flutter submodule
       # We can't do a depth-1 clone, because we need the most recent tag
       # so that Flutter knows its version and sees the constraint in our
       # pubspec is satisfied.  It's uncommon for flutter/flutter to go
-      # more than 100 commits between tags.  Fetch 1000 for good measure.
+      # more than 100 commits between tags. Fetch 1000 for good measure.
+      # This is also why we don't use the `with submodules` in the
+      # `checkout` action.
       run: |
-        git clone --depth=1000 https://github.com/flutter/flutter ~/flutter
-        TZ=UTC git --git-dir ~/flutter/.git log -1 --format='%h | %ci | %s' --date=iso8601-local
-        echo ~/flutter/bin >> "$GITHUB_PATH"
+        git submodule update --init --depth 1000
+
+    - name: Add vendored flutter to PATH
+      run: |
+        echo vendor/flutter/bin >> "$GITHUB_PATH"
+
+    - name: Display version (Temporary)
+      run: |
+        flutter --version
 
     - name: Download Flutter SDK artifacts (flutter precache)
       run: flutter precache --universal

.gitmodules

@@ -0,0 +1,4 @@
+[submodule "vendor/flutter"]
+	path = vendor/flutter
+	url = https://github.com/flutter/flutter.git
+	branch = main

.vscode/settings.json

@@ -17,4 +17,5 @@
 
   // This much more focused automatic fix is helpful, though.
   "files.trimTrailingWhitespace": true,
+  "dart.flutterSdkPaths": ["vendor/flutter"],
 }

README.md

@@ -92,26 +92,21 @@ Two specific points to expand on:
 
 ### Setting up
 
-1. Follow the [Flutter installation guide](https://docs.flutter.dev/get-started/install)
-   for your platform of choice.
-2. Switch to the latest version of Flutter by running `flutter channel main`
-   and `flutter upgrade` (see [Flutter version](#flutter-version) below).
+1. Install direnv (TBA), and set up its hook.
+2. Make sure you have initialized submodules by doing (do
+not do `--depth=1` as flutter relies on tags to know its version):
+
+  ```sh
+  git submodule update --init
+  ```
 3. Ensure Flutter is correctly configured by running `flutter doctor`.
 4. Start the app with `flutter run`, or from your IDE.
 
 
 ### Flutter version
 
-While in the beta phase, we use the latest Flutter from Flutter's
-main branch.  Use `flutter channel main` and `flutter upgrade`.
-
-We don't pin a specific version, because Flutter itself doesn't offer
-a way to do so.  So far that hasn't been a problem.  When it becomes one,
-we'll figure it out; there are several tools for this in the Flutter
-community.  See [issue #15][].
-
-[issue #15]: https://github.com/zulip/zulip-flutter/issues/15
-
+We pin to a particular version of flutter SDK via git submodules in [vendor/flutter/](vendor/flutter/).
+If your local checkout of this repository does not have this submodule checked out at the same commit, the build may fail.
 
 ### Tests
 
@@ -242,13 +237,16 @@ that's a good prompt to do this.  We also do this when there's a
 new PR merged that we particularly want to take.
 
 To update the version bounds:
+* First, make sure you're on the `main` channel, run:
+  `flutter channel main`.
 * Use `flutter upgrade` to upgrade your local Flutter and Dart.
 * Update the lower bounds at `environment` in `pubspec.yaml`
   to the new versions, as seen in `flutter --version`.
 * Run `flutter pub get`, which will update `pubspec.lock`.
 * Make a quick check that things work: `tools/check`,
   and do a quick smoke-test of the app.
-* Commit and push the changes in `pubspec.yaml` and `pubspec.lock`.
+* Commit and push the changes in the submodule `vendor/flutter`,
+  `pubspec.yaml` and `pubspec.lock`.
 
 
 ### Upgrading dependencies

analysis_options.yaml

@@ -9,6 +9,9 @@
 # packages, and plugins designed to encourage good coding practices.
 include: package:flutter_lints/flutter.yaml
 
+analyzer:
+  exclude: [vendor/**]
+
 linter:
   # The lint rules applied to this project can be customized in the
   # section below to disable rules from the `package:flutter_lints/flutter.yaml`