Upgrade null safety example (#748)

null_safety/README.md

@@ -0,0 +1,30 @@
+# Null safety Flutter example
+
+This folder contains a demonstration of Dart's sound null
+safety feature. For additional information about this feature, see
+https://dart.dev/null-safety.
+
+The folder contains two sample Flutter apps:
+
+  * `null_unsafe_app`: This contains a small weather app, which has three null
+    errors in its rendering code as a result of not using null safety.
+    
+    Try running the app (`flutter run` in a terminal, or the Run action in
+    VSCode/Android Studio), and see if the UI works. The null errors are
+    sporadic, so try hot reloading a bunch of times and experience the app
+    sometimes working and sometimes failing.
+
+  * `null_safe_app`: This contains the same weather app, but this time using
+    null safety. There are two copies of the source code:
+
+    `lib/badapp.dart`: This is the same source code as in the unsafe app. Try
+    uncommenting the two commented lines of null-unsafe code, and notice how the
+    Dart analyzer with null safety immediately flags the lines as unsafe.
+
+    `lib/goodapp.dart`: This a safe version of the unsafe app, with the null
+    errors fixed. Read through the comments to see illustrations of how to resolve
+    null errors with null safety.
+
+## Running the example code
+
+This sample requires Flutter 2.0 or later.