Temporarily run Windows tests via GitHub Actions (flutter#3875)

Until the infrastructure limitations that led to flutter#82032 are fixed, run basic Windows tests via GitHub Actions (which do have VS available) so that we aren't missing all Windows coverage.

.github/workflows/windows_ci.yml

@@ -0,0 +1,50 @@
+name: CI
+
+on: [push, pull_request]
+
+jobs:
+  build_and_drive_windows:
+    runs-on: windows-2019
+    strategy:
+      matrix:
+        channel: ['master', 'stable']
+      # Disable fail-fast to get results from both channels even if one fails.
+      fail-fast: false
+
+    steps:
+    - uses: actions/checkout@5a4ac9002d0be2fb38bd78e4b4dbde5606d7042f
+      with:
+        path: plugins
+    - uses: actions/checkout@5a4ac9002d0be2fb38bd78e4b4dbde5606d7042f
+      with:
+        path: flutter
+        repository: flutter/flutter
+        ref: master
+        # Shallow clones don't work; see https://github.com/flutter/flutter/issues/18532
+        fetch-depth: 0
+    - name: Set up Flutter
+      run: |
+        # Add tags, which are also necessary for version checks to work.
+        git fetch origin +refs/tags/*:refs/tags/*
+        # Set channel
+        bin\flutter channel ${{matrix.channel}}
+        bin\flutter upgrade
+        # Enable desktop
+        bin\flutter config --enable-windows-desktop
+        # Add fluter to path.
+        echo "$env:GITHUB_WORKSPACE\flutter\bin" | Out-File -FilePath $env:GITHUB_PATH -Encoding utf8 -Append
+      working-directory: ${{ github.workspace }}/flutter
+    - name: Doctor
+      # Run doctor, for ease of debugging any issues.
+      run: flutter doctor -v
+    - name: Set up tools
+      run: dart pub get
+      working-directory: ${{ github.workspace }}/plugins/script/tool
+    - name: Build
+      run: |
+        dart run ./script/tool/lib/src/main.dart build-examples --windows --run-on-changed-packages
+      working-directory: ${{ github.workspace }}/plugins
+    - name: Drive
+      run: |
+        dart run ./script/tool/lib/src/main.dart drive-examples --windows --run-on-changed-packages
+      working-directory: ${{ github.workspace }}/plugins

script/tool/lib/src/firebase_test_lab_command.dart

@@ -27,8 +27,8 @@ class FirebaseTestLabCommand extends PluginCommand {
       help: 'The Firebase project name.',
     );
     argParser.addOption('service-key',
-        defaultsTo:
-            p.join(io.Platform.environment['HOME'], 'gcloud-service-key.json'));
+        defaultsTo: p.join(
+            io.Platform.environment['HOME'] ?? '/', 'gcloud-service-key.json'));
     argParser.addOption('test-run-id',
         defaultsTo: Uuid().v4(),
         help: