Test `flutter create --offline` (#23278)

Verifies that flutter tools isn't rebuilt. Doesn't truly check that the
created output is runnable.

dev/devicelab/bin/tasks/flutter_create_offline_test_linux.dart

+import 'package:flutter_devicelab/framework/framework.dart';
+import 'package:flutter_devicelab/tasks/integration_tests.dart';
+
+Future<void> main() async {
+  await task(createFlutterCreateOfflineTest());
+}

dev/devicelab/bin/tasks/flutter_create_offline_test_mac.dart

+import 'package:flutter_devicelab/framework/framework.dart';
+import 'package:flutter_devicelab/tasks/integration_tests.dart';
+
+Future<void> main() async {
+  await task(createFlutterCreateOfflineTest());
+}

dev/devicelab/bin/tasks/flutter_create_offline_test_windows.dart

+import 'package:flutter_devicelab/framework/framework.dart';
+import 'package:flutter_devicelab/tasks/integration_tests.dart';
+
+Future<void> main() async {
+  await task(createFlutterCreateOfflineTest());
+}

dev/devicelab/lib/tasks/integration_tests.dart

@@ -3,7 +3,9 @@
 // found in the LICENSE file.
 
 import 'dart:async';
+import 'dart:io';
 
+import 'package:path/path.dart' as path;
 import '../framework/adb.dart';
 import '../framework/framework.dart';
 import '../framework/ios.dart';
@@ -59,6 +61,22 @@ TaskFunction createAndroidSemanticsIntegrationTest() {
   );
 }
 
+TaskFunction createFlutterCreateOfflineTest() {
+  return () async {
+    final Directory tempDir = Directory.systemTemp.createTempSync('flutter_create_test.');
+    String output;
+    await inDirectory(tempDir, () async {
+      output = await eval(path.join(flutterDirectory.path, 'bin', 'flutter'), <String>['create', '--offline', 'flutter_create_test']);
+    });
+    if (output.contains(RegExp('building flutter tool', caseSensitive: false))) {
+      return TaskResult.failure('`flutter create --offline` should not rebuild flutter tool');
+    } else if (!output.contains('All done!')) {
+      return TaskResult.failure('`flutter create` failed');
+    }
+    return TaskResult.success(null);
+  };
+}
+
 class DriverTest {
 
   DriverTest(

dev/devicelab/manifest.yaml

@@ -283,6 +283,27 @@ tasks:
     stage: devicelab
     required_agent_capabilities: ["linux/android"]
 
+  flutter_create_offline_test_linux:
+    description: >
+      Tests the `flutter create --offline` command.
+    stage: devicelab
+    required_agent_capabilities: ["linux/android"]
+    flaky: true
+
+  flutter_create_offline_test_windows:
+    description: >
+      Tests the `flutter create --offline` command.
+    stage: devicelab
+    required_agent_capabilities: ["windows/android"]
+    flaky: true
+
+  flutter_create_offline_test_mac:
+    description: >
+      Tests the `flutter create --offline` command.
+    stage: devicelab
+    required_agent_capabilities: ["mac/android"]
+    flaky: true
+
   # iOS on-device tests
 
   flavors_test_ios: