[ Tool ] Fix missing import for widget_preview.dart (flutter#173731)

The import was removed in a PR that landed after all presubmits passed.

Author: bkonyi

packages/flutter_tools/lib/src/commands/widget_preview.dart

@@ -27,6 +27,7 @@ import '../isolated/resident_web_runner.dart';
 import '../project.dart';
 import '../resident_runner.dart';
 import '../runner/flutter_command.dart';
+import '../runner/flutter_command_runner.dart';
 import '../web/web_device.dart';
 import '../widget_preview/analytics.dart';
 import '../widget_preview/dependency_graph.dart';