Use the correct set of options to analyze Flutter packages. (#9758)

This affects the Fuchsia build which will soon run analysis by default.
diff --git a/examples/flutter_gallery/BUILD.gn b/examples/flutter_gallery/BUILD.gn
index 74d74f2..eff7bb8 100644
--- a/examples/flutter_gallery/BUILD.gn
+++ b/examples/flutter_gallery/BUILD.gn
@@ -9,6 +9,8 @@
 flutter_app("flutter_gallery") {
   main_dart = "lib/main.dart"
 
+  analysis_options = "//lib/flutter/.analysis_options"
+
   deps = [
     "//lib/flutter/packages/flutter",
   ]