Add the infinite_list sample (#440)

This PR adds a Flutter sample app that shows an implementation of the "infinite list" UX pattern. That is, a list is shown to the user as if it was continuous although it is internally paginated. This is a common feature of mobile apps, from shopping catalogs through search engines to social media clients.

infinite_list/pubspec.yaml

@@ -0,0 +1,25 @@
+name: infinitelist
+description: >
+  A sample implementation of an infinite list.
+
+version: 1.0.0+1
+
+environment:
+  sdk: ">=2.6.0 <3.0.0"
+
+dependencies:
+  flutter:
+    sdk: flutter
+
+  cupertino_icons: ^0.1.3
+  meta: ^1.1.8
+  provider: ^4.0.0
+
+dev_dependencies:
+  flutter_test:
+    sdk: flutter
+
+  pedantic: ^1.9.0
+
+flutter:
+  uses-material-design: true