Add a new null safety example (#562)

experimental/null_safety/null_safe_app/.vscode/launch.json

@@ -0,0 +1,17 @@
+{
+    // Use IntelliSense to learn about possible attributes.
+    // Hover to view descriptions of existing attributes.
+    // For more information, visit: https://go.microsoft.com/fwlink/?linkid=830387
+    "version": "0.2.0",
+    "configurations": [
+        {
+            "name": "null_safe_app",
+            "program": "lib/main.dart",
+            "request": "launch",
+            "type": "dart",
+            "args": [
+                "--enable-experiment=non-nullable",
+            ],
+        }
+    ]
+}