Turn back on macOS shard Cirrus caching (#50496)

.cirrus.yml

@@ -438,8 +438,6 @@ task:
         - dart --enable-asserts dev\customer_testing\run_tests.dart --skip-on-fetch-failure --skip-template bin/cache/pkg/tests/registry/*.test
 
 # MACOS SHARDS
-# Mac doesn't use caches because they apparently take longer to populate and save
-# than just fetching the data in the first place.
 task:
   osx_instance:
     image: catalina-flutter # see https://cirrus-ci.org/guide/macOS/ for list of images (we should update regularly)
@@ -452,6 +450,21 @@ task:
     COCOAPODS_DISABLE_STATS: true
     CPU: 2
     MEMORY: 8G
+  all_flutter_cache:
+    folder: bin/cache/
+    fingerprint_script: echo $OS; cat bin/internal/*.version
+    populate_script:
+      - git fetch origin
+      - git fetch origin master # To set FETCH_HEAD, so that "git merge-base" works.
+      - flutter precache
+      - flutter doctor -v
+  pub_cache:
+    folder: $HOME/.pub-cache
+    fingerprint_script: echo $OS; grep -r --include=pubspec.yaml 'PUBSPEC CHECKSUM' "$CIRRUS_WORKING_DIR"
+    populate_script:
+      - git fetch origin
+      - git fetch origin master # To set FETCH_HEAD, so that "git merge-base" works.
+      - flutter update-packages
   setup_script:
     - date
     - which flutter
@@ -463,7 +476,7 @@ task:
     - git fetch origin master # To set FETCH_HEAD, so that "git merge-base" works.
     - flutter config --no-analytics
     - flutter doctor -v
-    - flutter update-packages
+    - flutter update-packages --offline
     - date
     - which flutter
   on_failure:
@@ -575,7 +588,9 @@ task:
       script:
         - ulimit -S -n 2048 # https://github.com/flutter/flutter/issues/2976
         - dart --enable-asserts ./dev/bots/codesign.dart
-
+  cleanup_before_cache_upload_script:
+     # Remove volatile file to avoid unnecessary Cirrus cache invalidation.
+    - rm -f bin/cache/flutter_version_check.stamp
 docker_builder:
   # Only build a new docker image when we tag a release (for dev, beta, or
   # stable). Note: tagging a commit and pushing to a release branch are

packages/flutter_tools/lib/src/commands/update_packages.dart

@@ -75,6 +75,12 @@ class UpdatePackagesCommand extends FlutterCommand {
         help: 'verifies the package checksum without changing or updating deps',
         defaultsTo: false,
         negatable: false,
+      )
+      ..addFlag(
+        'offline',
+        help: 'Use cached packages instead of accessing the network',
+        defaultsTo: false,
+        negatable: false,
       );
   }
 
@@ -129,6 +135,13 @@ class UpdatePackagesCommand extends FlutterCommand {
     final bool isPrintTransitiveClosure = boolArg('transitive-closure');
     final bool isVerifyOnly = boolArg('verify-only');
     final bool isConsumerOnly = boolArg('consumer-only');
+    final bool offline = boolArg('offline');
+
+    if (upgrade && offline) {
+      throwToolExit(
+          '--force-upgrade cannot be used with the --offline flag'
+      );
+    }
 
     // "consumer" packages are those that constitute our public API (e.g. flutter, flutter_test, flutter_driver, flutter_localizations).
     if (isConsumerOnly) {
@@ -268,6 +281,7 @@ class UpdatePackagesCommand extends FlutterCommand {
           directory: tempDir.path,
           upgrade: true,
           checkLastModified: false,
+          offline: offline,
         );
         // Then we run "pub deps --style=compact" on the result. We pipe all the
         // output to tree.fill(), which parses it so that it can create a graph
@@ -335,7 +349,12 @@ class UpdatePackagesCommand extends FlutterCommand {
     int count = 0;
 
     for (final Directory dir in packages) {
-      await pub.get(context: PubContext.updatePackages, directory: dir.path, checkLastModified: false);
+      await pub.get(
+        context: PubContext.updatePackages,
+        directory: dir.path,
+        checkLastModified: false,
+        offline: offline,
+      );
       count += 1;
     }