Add a new null safety example (#562)

experimental/null_safety/README.md

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+# Null safety Flutter example
+
+This folder contains a demonstration of the upcoming Dart feature, sound null
+safety. 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
+
+Null safety is still in technical preview. As a result you need to use a
+dev-channel Flutter build, and you have to pass an [experiment
+flag](https://dart.dev/tools/experiment-flags).
+
+### Flutter SDK requirements
+
+This sample is
+[experimental](https://github.com/flutter/samples/tree/master/experimental), and
+requires using a **dev channel** Flutter SDK. You can change to the dev channel
+with `flutte channel dev` followed by `flutter upgrade` (don't forget to change
+back to `flutter channel stable` for production use.
+
+### Running from the terminal/command-prompt
+
+Because null safety is still in tech preview, we need to pass a so-called
+_experiment flag_ when invoking any Flutter command in the terminal, which looks
+like this: `--enable-experiment=non-nullable`.
+
+To run the main app, type these commands in the terminal/command-prompt:
+
+  - Windows:
+    - `cd <folder with samples repo>\experimental\null_safety\null_safe_app\`
+    - `flutter pub get`
+    - `flutter run --enable-experiment=non-nullable` 
+  - macOS/Linux:
+    - `cd <folder with samples repo>/experimental/null_safety/null_safe_app/`
+    - `flutter pub get`
+    - `flutter run --enable-experiment=non-nullable`
+
+### Running from VSCode
+
+This example contains a launch configuration for VSCode that passes the
+experimental flag, so to run the sample in VSCode:
+
+  1. Invoke **File > Open**, and select the `null_safety/null_safe_app` folder.
+
+  1. Select **Run > Run** and the project should run and print a message in the
+     Debug Console.
+
+
+### Running from Android Studio
+
+  1. Start Android Studio.
+
+  1. Select Open Project, and select the `null_safety/null_safe_app` folder.
+
+  1. Open the file `lib/main.dart` in the code editor.
+
+  1. Select Run > Run and the project should run and print a message in the Run
+     pane.