Minor adjustments to Cirrus configuration (#336)

- Use a specific version of the plugin tools, in preparation for
  publishing an updated version out of flutter/plugins.
- Use templates the same way we do in flutter/plugins for consistency
  and ease of maintenance.
- Separate some setup steps into their own script steps for better
  inspectability in the UI.
- Enable build tests on stable

.cirrus.yml

@@ -1,5 +1,22 @@
 gcp_credentials: ENCRYPTED[1816835da1e936dabb469b51501856ec8468676e35e967bd0fd720a815498e5ee6c8a6a79219ce273f67bcb8f1aa948a]
 
+env:
+  CHANNEL: "master" # Default to master when not explicitly set by a task.
+
+tool_setup_template: &TOOL_SETUP_TEMPLATE
+  tool_setup_script:
+    - git fetch origin master # To set FETCH_HEAD for "git merge-base" to work
+    # Pinned version of the plugin tools, to avoid breakage in this repository
+    # when pushing updates from flutter/plugins.
+    - pub global activate flutter_plugin_tools 0.0.45+3
+
+flutter_upgrade_template: &FLUTTER_UPGRADE_TEMPLATE
+  upgrade_flutter_script:
+    - flutter channel $CHANNEL
+    - flutter upgrade
+    - flutter doctor
+  << : *TOOL_SETUP_TEMPLATE
+
 task:
   gke_container:
     dockerfile: .ci/Dockerfile
@@ -10,12 +27,7 @@ task:
     namespace: default
     cpu: 4
     memory: 8G
-  upgrade_script:
-    - flutter channel master
-    - flutter upgrade
-    - flutter doctor
-    - git fetch origin master
-  activate_script: pub global activate flutter_plugin_tools
+  << : *FLUTTER_UPGRADE_TEMPLATE
   matrix:
     - name: analyze
       script: ./script/incremental_build.sh analyze --custom-analysis=web_benchmarks/testing/test_app
@@ -30,6 +42,9 @@ task:
         - analyze
     - name: build-apks+java-test
       env:
+        matrix:
+          CHANNEL: "master"
+          CHANNEL: "stable"
         matrix:
           BUILD_SHARDING: "--shardIndex 0 --shardCount 2"
           BUILD_SHARDING: "--shardIndex 1 --shardCount 2"
@@ -39,10 +54,10 @@ task:
       depends_on:
         - analyze
     - name: web_benchmarks_test
-      script:
+      install_chromium_script:
         - ./script/install_chromium.sh
+      script:
         - export CHROME_EXECUTABLE=$(pwd)/.chromium/chrome-linux/chrome
-        - flutter config --enable-web
         - cd packages/web_benchmarks/testing/test_app
         - flutter packages get
         - cd ../..
@@ -56,15 +71,13 @@ task:
     image: big-sur-xcode-12.4
   env:
     PATH: $PATH:/usr/local/bin
+    matrix:
+      CHANNEL: "master"
+      CHANNEL: "stable"
     matrix:
       BUILD_SHARDING: "--shardIndex 0 --shardCount 2"
       BUILD_SHARDING: "--shardIndex 1 --shardCount 2"
-  setup_script:
-    - flutter channel master
-    - flutter upgrade
-    - flutter doctor
-    - git fetch origin master
-    - pub global activate flutter_plugin_tools
+  << : *FLUTTER_UPGRADE_TEMPLATE
   build_script:
     - ./script/incremental_build.sh build-examples --ipa
 
@@ -78,13 +91,8 @@ task:
     matrix:
       CHANNEL: "master"
       CHANNEL: "stable"
-  setup_script:
-    - git fetch origin master
-    - pub global activate flutter_plugin_tools
+  << : *FLUTTER_UPGRADE_TEMPLATE
+  clang_format_setup_script:
     - brew install clang-format
-  upgrade_script:
-    - flutter channel $CHANNEL
-    - flutter upgrade
-    - flutter doctor
   build_script:
     - ./script/local_tests.sh