Streamline CI setup, and reenable macOS credits (flutter#3697)

This makes a few improvements to CI throughput:
- Moves the flutter channel+upgrade step into each step, rather then doing it for both channels in the setup step. This avoids pointlessly doing twice the work in every setup (downloading two channels, only one of which will be used)
- Re-enables credits for macOS tasks. These were disabled in a recent chance since I was unifying configurations, but it turns out that without credits there is *no* parallelization of macOS tasks, which makes the end-to-end time very long.
- Dials back the Linux VM requirements based on looking at the new CPU and memory usage graphs for runs:
  - No task was ever reaching 4 CPUs or 12 GB of memory, so use those values for heavy workload; this will double concurrency of the heavy-workload tasks and should have no impact on run length.
  - Linux build+drive was extremely low usage, so move it to the light workload group.

Author: stuartmorgan-g

.cirrus.yml

@@ -1,21 +1,24 @@
-# Light-workload tasks.
-# These use default machines, with fewer CPUs, to reduce pressure on the
-# concurrency limits.
-task:
-  # don't run on release tags since it creates O(n^2) tasks where n is the number of plugins
+root_task_template: &ROOT_TASK_TEMPLATE
+  # Don't run on release tags since it creates O(n^2) tasks where n is the
+  # number of plugins
   only_if: $CIRRUS_TAG == ''
   use_compute_credits: $CIRRUS_USER_COLLABORATOR == 'true' && $CIRRUS_PR == ''
-  container:
-    dockerfile: .ci/Dockerfile
   env:
     INTEGRATION_TEST_PATH: "./packages/integration_test"
-  upgrade_script:
-    - flutter channel stable
-    - flutter upgrade
-    - flutter channel master
+    CHANNEL: "master" # Default to master when not explicitly set by a task.
+  setup_script:
+    - flutter channel $CHANNEL
     - flutter upgrade
-    - flutter config --enable-linux-desktop
-    - git fetch origin master
+    - git fetch origin master # To set FETCH_HEAD for "git merge-base" to work
+
+
+# Light-workload Linux tasks.
+# These use default machines, with fewer CPUs, to reduce pressure on the
+# concurrency limits.
+task:
+  << : *ROOT_TASK_TEMPLATE
+  container:
+    dockerfile: .ci/Dockerfile
   matrix:
     ### Platform-agnostic tasks ###
     - name: plugin_tools_tests
@@ -25,7 +28,6 @@ task:
         - CIRRUS_BUILD_ID=null pub run test
     - name: publishable
       script:
-        - flutter channel master
         - ./script/check_publish.sh
     - name: format
       format_script: ./script/incremental_build.sh format --fail-on-change
@@ -35,14 +37,12 @@ task:
           CHANNEL: "master"
           CHANNEL: "stable"
       test_script:
-        - flutter channel $CHANNEL
         - ./script/incremental_build.sh test
     - name: analyze_master
       env:
         matrix:
           CHANNEL: "master"
       script:
-        - flutter channel $CHANNEL
         - ./script/incremental_build.sh analyze
     ## TODO(cyanglaz):
     ## Combing stable and master analyze jobs when integration test null safety is ready on flutter stable.
@@ -51,7 +51,6 @@ task:
         matrix:
           CHANNEL: "stable"
       script:
-        - flutter channel $CHANNEL
         - find . -depth -type d -wholename '*_web/example' -exec rm -rf {} \;
         - ./script/incremental_build.sh analyze
     ### Android tasks ###
@@ -61,7 +60,6 @@ task:
           CHANNEL: "master"
           CHANNEL: "stable"
       script:
-        - flutter channel $CHANNEL
         - ./script/build_all_plugins_app.sh apk
     ### Web tasks ###
     - name: build_all_plugins_web
@@ -70,14 +68,12 @@ task:
           CHANNEL: "master"
           CHANNEL: "stable"
       script:
-        - flutter channel $CHANNEL
         - ./script/build_all_plugins_app.sh web
     - name: build-web-examples
       env:
         matrix:
           CHANNEL: "master"
       build_script:
-        - flutter channel $CHANNEL
         - ./script/incremental_build.sh build-examples --web
         # TODO: Add driving examples (and move to heavy-workload group).
     ### Linux desktop tasks ###
@@ -87,8 +83,17 @@ task:
           CHANNEL: "master"
           CHANNEL: "stable"
       script:
-        - flutter channel $CHANNEL
+        - flutter config --enable-linux-desktop
         - ./script/build_all_plugins_app.sh linux
+    - name: build-linux+drive-examples
+      env:
+        matrix:
+          CHANNEL: "master"
+          CHANNEL: "stable"
+      build_script:
+        - flutter config --enable-linux-desktop
+        - ./script/incremental_build.sh build-examples --linux
+        - xvfb-run ./script/incremental_build.sh drive-examples --linux
 
 # Legacy Dockerfile configuration for web integration tests.
 # https://github.com/flutter/web_installers doesn't yet support the current
@@ -97,20 +102,9 @@ task:
 # tasks" block above once web_installers has been updated to support Chrome 89
 # (which is what the current image generated from .ci/Dockerfile has).
 task:
-  # don't run on release tags since it creates O(n^2) tasks where n is the number of plugins
-  only_if: $CIRRUS_TAG == ''
-  use_compute_credits: $CIRRUS_USER_COLLABORATOR == 'true' && $CIRRUS_PR == ''
+  << : *ROOT_TASK_TEMPLATE
   container:
     dockerfile: .ci/Dockerfile-LegacyChrome
-  env:
-    INTEGRATION_TEST_PATH: "./packages/integration_test"
-  upgrade_script:
-    - flutter channel stable
-    - flutter upgrade
-    - flutter channel master
-    - flutter upgrade
-    - flutter config --enable-linux-desktop
-    - git fetch origin master
   matrix:
     - name: integration_web_smoke_test
       env:
@@ -120,7 +114,6 @@ task:
       # Tests integration example test in web.
       only_if: "changesInclude('.cirrus.yml', 'packages/integration_test/**') || $CIRRUS_PR == ''"
       install_script:
-        - flutter channel $CHANNEL
         - git clone https://github.com/flutter/web_installers.git
         - cd web_installers/packages/web_drivers/
         - pub get
@@ -130,26 +123,15 @@ task:
         - cd $INTEGRATION_TEST_PATH/example/
         - flutter drive -v --driver=test_driver/integration_test.dart --target=integration_test/example_test.dart -d web-server --release --browser-name=chrome
 
-# Heavy-workload tasks.
+# Heavy-workload Linux tasks.
 # These use machines with more CPUs and memory, so will reduce parallelization
 # for non-credit runs.
 task:
-  # don't run on release tags since it creates O(n^2) tasks where n is the number of plugins
-  only_if: $CIRRUS_TAG == ''
-  use_compute_credits: $CIRRUS_USER_COLLABORATOR == 'true' && $CIRRUS_PR == ''
+  << : *ROOT_TASK_TEMPLATE
   container:
     dockerfile: .ci/Dockerfile
-    cpu: 8
-    memory: 16G
-  env:
-    INTEGRATION_TEST_PATH: "./packages/integration_test"
-  upgrade_script:
-    - flutter channel stable
-    - flutter upgrade
-    - flutter channel master
-    - flutter upgrade
-    - flutter config --enable-linux-desktop
-    - git fetch origin master
+    cpu: 4
+    memory: 12G
   matrix:
     ### Android tasks ###
     - name: build-apks+java-test+firebase-test-lab
@@ -165,7 +147,6 @@ task:
         MAPS_API_KEY: ENCRYPTED[596a9f6bca436694625ac50851dc5da6b4d34cba8025f7db5bc9465142e8cd44e15f69e3507787753accebfc4910d550]
         GCLOUD_FIREBASE_TESTLAB_KEY: ENCRYPTED[07586610af1fdfc894e5969f70ef2458341b9b7e9c3b7c4225a663b4a48732b7208a4d91c3b7d45305a6b55fa2a37fc4]
       script:
-        - flutter channel $CHANNEL
         # Unsetting CIRRUS_CHANGE_MESSAGE and CIRRUS_COMMIT_MESSAGE as they
         # might include non-ASCII characters which makes Gradle crash.
         # See: https://github.com/flutter/flutter/issues/24935
@@ -186,35 +167,16 @@ task:
         - fi
         - export CIRRUS_CHANGE_MESSAGE=`cat /tmp/cirrus_change_message.txt`
         - export CIRRUS_COMMIT_MESSAGE=`cat /tmp/cirrus_commit_message.txt`
-    ### Linux desktop tasks ###
-    - name: build-linux+drive-examples
-      env:
-        matrix:
-          CHANNEL: "master"
-          CHANNEL: "stable"
-      build_script:
-        - flutter channel $CHANNEL
-        - ./script/incremental_build.sh build-examples --linux
-        - xvfb-run ./script/incremental_build.sh drive-examples --linux
 
+# macOS tasks.
 task:
-  # Xcode 12 task
-  # don't run on release tags since it creates O(n^2) tasks where n is the number of plugins
-  only_if: $CIRRUS_TAG == ''
-  use_compute_credits: $CIRRUS_USER_COLLABORATOR == 'true' && $CIRRUS_PR == ''
+  << : *ROOT_TASK_TEMPLATE
+  # Only one macOS task can run in parallel without credits, so use them for
+  # PRs on macOS.
+  use_compute_credits: $CIRRUS_USER_COLLABORATOR == 'true'
   osx_instance:
     image: big-sur-xcode-12.3
-  upgrade_script:
-    - sudo gem install cocoapods
-    - flutter channel stable
-    - flutter upgrade
-    - flutter channel master
-    - flutter upgrade
-    - flutter config --enable-macos-desktop
-    - git fetch origin master
-  create_simulator_script:
-    - xcrun simctl list
-    - xcrun simctl create Flutter-iPhone com.apple.CoreSimulator.SimDeviceType.iPhone-11 com.apple.CoreSimulator.SimRuntime.iOS-14-3 | xargs xcrun simctl boot
+  cocoapod_install_script: sudo gem install cocoapods
   matrix:
     ### Platform-agnostic tasks ###
     - name: lint_darwin_plugins
@@ -225,8 +187,6 @@ task:
       script:
         # TODO(jmagman): Lint macOS podspecs but skip any that fail library validation.
         - find . -name "*.podspec" | xargs grep -l "osx" | xargs rm
-        # Skip the dummy podspecs used to placate the tool.
-        - find . -name "*_web*.podspec" -o -name "*_mac*.podspec" | xargs rm
         - ./script/incremental_build.sh podspecs
     ### iOS tasks ###
     - name: build_all_plugins_ipa
@@ -235,7 +195,6 @@ task:
           CHANNEL: "master"
           CHANNEL: "stable"
       script:
-        - flutter channel $CHANNEL
         - ./script/build_all_plugins_app.sh ios --no-codesign
     - name: build-ipas+drive-examples
       env:
@@ -250,8 +209,10 @@ task:
           CHANNEL: "master"
           CHANNEL: "stable"
         SIMCTL_CHILD_MAPS_API_KEY: ENCRYPTED[596a9f6bca436694625ac50851dc5da6b4d34cba8025f7db5bc9465142e8cd44e15f69e3507787753accebfc4910d550]
+      create_simulator_script:
+        - xcrun simctl list
+        - xcrun simctl create Flutter-iPhone com.apple.CoreSimulator.SimDeviceType.iPhone-11 com.apple.CoreSimulator.SimRuntime.iOS-14-3 | xargs xcrun simctl boot
       build_script:
-        - flutter channel $CHANNEL
         - ./script/incremental_build.sh build-examples --ipa
         - ./script/incremental_build.sh xctest --skip $PLUGINS_TO_SKIP_XCTESTS --ios-destination "platform=iOS Simulator,name=iPhone 11,OS=latest"
         # `drive-examples` contains integration tests, which changes the UI of the application.
@@ -265,7 +226,7 @@ task:
           CHANNEL: "master"
           CHANNEL: "stable"
       script:
-        - flutter channel $CHANNEL
+        - flutter config --enable-macos-desktop
         - ./script/build_all_plugins_app.sh macos
     - name: build-macos+drive-examples
       env:
@@ -274,6 +235,6 @@ task:
           CHANNEL: "stable"
         PATH: $PATH:/usr/local/bin
       build_script:
-        - flutter channel $CHANNEL
+        - flutter config --enable-macos-desktop
         - ./script/incremental_build.sh build-examples --macos --no-ipa
         - ./script/incremental_build.sh drive-examples --macos