[Reland] Fix flavors test install checks (#134060)

Reland of https://github.com/flutter/flutter/pull/133719. Updates [the string comparison in flavor_test.dart](https://github.com/flutter/flutter/pull/134060/files#diff-53383b32b975bfed6875306dfb98911cad077a5251ca0591c5b0e125fb4a0f05R39) to use `path.join` to build the path string so that the generated path is correct for both Linux and Windows hosts.

Fixes https://github.com/flutter/flutter/issues/133713

I've tested this on a Windows host targeting a physical Android device.

dev/devicelab/bin/tasks/flavors_test.dart

@@ -7,37 +7,45 @@ import 'package:flutter_devicelab/framework/framework.dart';
 import 'package:flutter_devicelab/framework/task_result.dart';
 import 'package:flutter_devicelab/framework/utils.dart';
 import 'package:flutter_devicelab/tasks/integration_tests.dart';
+import 'package:path/path.dart' as path;
 
 Future<void> main() async {
   deviceOperatingSystem = DeviceOperatingSystem.android;
   await task(() async {
     await createFlavorsTest().call();
     await createIntegrationTestFlavorsTest().call();
-    // test install and uninstall of flavors app
-    await inDirectory('${flutterDirectory.path}/dev/integration_tests/flavors', () async {
-      await flutter(
-        'install',
-        options: <String>['--debug', '--flavor', 'paid'],
-      );
-      await flutter(
-        'install',
-        options: <String>['--debug', '--flavor', 'paid', '--uninstall-only'],
-      );
-      final StringBuffer stderr = StringBuffer();
-      await evalFlutter(
-        'install',
-        canFail: true,
-        stderr: stderr,
-        options: <String>['--flavor', 'bogus'],
-      );
 
-      final String stderrString = stderr.toString();
-      if (!stderrString.contains('The Xcode project defines schemes: free, paid')) {
-        print(stderrString);
-        return TaskResult.failure('Should not succeed with bogus flavor');
-      }
-    });
+    final TaskResult installTestsResult = await inDirectory(
+      '${flutterDirectory.path}/dev/integration_tests/flavors',
+      () async {
+        await flutter(
+          'install',
+          options: <String>['--debug', '--flavor', 'paid'],
+        );
+        await flutter(
+          'install',
+          options: <String>['--debug', '--flavor', 'paid', '--uninstall-only'],
+        );
 
-    return TaskResult.success(null);
+        final StringBuffer stderr = StringBuffer();
+        await evalFlutter(
+          'install',
+          canFail: true,
+          stderr: stderr,
+          options: <String>['--flavor', 'bogus'],
+        );
+
+        final String stderrString = stderr.toString();
+        final String expectedApkPath = path.join('build', 'app', 'outputs', 'flutter-apk', 'app-bogus-release.apk');
+        if (!stderrString.contains('"$expectedApkPath" does not exist.')) {
+          print(stderrString);
+          return TaskResult.failure('Should not succeed with bogus flavor');
+        }
+
+        return TaskResult.success(null);
+      },
+    );
+
+    return installTestsResult;
   });
 }

dev/devicelab/bin/tasks/flavors_test_ios.dart

@@ -14,30 +14,35 @@ Future<void> main() async {
     await createFlavorsTest().call();
     await createIntegrationTestFlavorsTest().call();
     // test install and uninstall of flavors app
-    await inDirectory('${flutterDirectory.path}/dev/integration_tests/flavors', () async {
-      await flutter(
-        'install',
-        options: <String>['--flavor', 'paid'],
-      );
-      await flutter(
-        'install',
-        options: <String>['--flavor', 'paid', '--uninstall-only'],
-      );
-      final StringBuffer stderr = StringBuffer();
-      await evalFlutter(
-        'install',
-        canFail: true,
-        stderr: stderr,
-        options: <String>['--flavor', 'bogus'],
-      );
+    final TaskResult installTestsResult = await inDirectory(
+      '${flutterDirectory.path}/dev/integration_tests/flavors',
+      () async {
+        await flutter(
+          'install',
+          options: <String>['--flavor', 'paid'],
+        );
+        await flutter(
+          'install',
+          options: <String>['--flavor', 'paid', '--uninstall-only'],
+        );
+        final StringBuffer stderr = StringBuffer();
+        await evalFlutter(
+          'install',
+          canFail: true,
+          stderr: stderr,
+          options: <String>['--flavor', 'bogus'],
+        );
 
-      final String stderrString = stderr.toString();
-      if (!stderrString.contains('install failed, bogus flavor not found')) {
-        print(stderrString);
-        return TaskResult.failure('Should not succeed with bogus flavor');
-      }
-    });
+        final String stderrString = stderr.toString();
+        if (!stderrString.contains('The Xcode project defines schemes: free, paid')) {
+          print(stderrString);
+          return TaskResult.failure('Should not succeed with bogus flavor');
+        }
 
-    return TaskResult.success(null);
+        return TaskResult.success(null);
+      },
+    );
+
+    return installTestsResult;
   });
 }

dev/devicelab/bin/tasks/flavors_test_macos.dart

@@ -14,26 +14,31 @@ Future<void> main() async {
     await createFlavorsTest().call();
     await createIntegrationTestFlavorsTest().call();
 
-    await inDirectory('${flutterDirectory.path}/dev/integration_tests/flavors', () async {
-      final StringBuffer stderr = StringBuffer();
+    final TaskResult installTestsResult = await inDirectory(
+      '${flutterDirectory.path}/dev/integration_tests/flavors',
+      () async {
+        final StringBuffer stderr = StringBuffer();
 
-      await evalFlutter(
-        'install',
-        canFail: true,
-        stderr: stderr,
-        options: <String>[
-          '--d', 'macos',
-          '--flavor', 'free'
-        ],
-      );
+        await evalFlutter(
+          'install',
+          canFail: true,
+          stderr: stderr,
+          options: <String>[
+            '--d', 'macos',
+            '--flavor', 'free'
+          ],
+        );
 
-      final String stderrString = stderr.toString();
-      if (!stderrString.contains('Host and target are the same. Nothing to install.')) {
-        print(stderrString);
-        return TaskResult.failure('Installing a macOS app on macOS should no-op');
-      }
-    });
+        final String stderrString = stderr.toString();
+        if (!stderrString.contains('Host and target are the same. Nothing to install.')) {
+          print(stderrString);
+          return TaskResult.failure('Installing a macOS app on macOS should no-op');
+        }
 
-    return TaskResult.success(null);
+        return TaskResult.success(null);
+      },
+    );
+
+    return installTestsResult;
   });
 }