Add an iOS style demo to the gallery (#12651)

* Built first tab

* Small additions

* started tab 3

* Need color arithmetics

* tab 2 built

* finalize

* lint and tests

* review

* Reapply docs after rebase

* use color.computeLuminance

* linter

* nit
diff --git a/examples/flutter_gallery/pubspec.yaml b/examples/flutter_gallery/pubspec.yaml
index 9d37894..18d18d4 100644
--- a/examples/flutter_gallery/pubspec.yaml
+++ b/examples/flutter_gallery/pubspec.yaml
@@ -6,12 +6,13 @@
   intl: 0.15.2
   string_scanner: 1.0.2
   url_launcher: 0.4.2+5
+  cupertino_icons: 0.1.1
 
   # Also update dev/benchmarks/complex_layout/pubspec.yaml
   flutter_gallery_assets:
     git:
       url: https://flutter.googlesource.com/gallery-assets
-      ref: 0b6cdb29b24ecc60781c2828086d955dacdc0152
+      ref: 6427f36af1bdb2fffaeda1c46d5e52d7b636c93d
 
 dev_dependencies:
   flutter_test:
@@ -137,6 +138,8 @@
     - packages/flutter_gallery_assets/shrine/vendors/peter-carlsson.png
     - packages/flutter_gallery_assets/shrine/vendors/sandra-adams.jpg
     - packages/flutter_gallery_assets/shrine/vendors/zach.jpg
+    - packages/flutter_gallery_assets/cupertino_navigation/person1.jpg
+    - packages/flutter_gallery_assets/cupertino_navigation/person2.jpg
   fonts:
     - family: Raleway
       fonts: