Add a DEMO banner to the layout details tab (#1389)

packages/devtools_app/lib/src/inspector/flutter/inspector_screen_details_tab.dart

diff --git a/packages/devtools_app/lib/src/inspector/flutter/inspector_screen_details_tab.dart b/packages/devtools_app/lib/src/inspector/flutter/inspector_screen_details_tab.dart
index 2675234..d3bb27b 100644
--- a/packages/devtools_app/lib/src/inspector/flutter/inspector_screen_details_tab.dart
+++ b/packages/devtools_app/lib/src/inspector/flutter/inspector_screen_details_tab.dart
@@ -43,7 +43,11 @@
     final tabViews = <Widget>[
       detailsTree,
       if (enableExperimentalStoryOfLayout)
-        LayoutDetailsTab(controller: controller),
+        Banner(
+          message: 'PROTOTYPE',
+          location: BannerLocation.topStart,
+          child: LayoutDetailsTab(controller: controller),
+        ),
     ];
     final focusColor = Theme.of(context).focusColor;
     return Container(