Follow up on null safety codelab (#3112)

* Add null safety codelab to the codelab index
* Listof -> ListOf
* Add link to issue #3093
* Retire the tour in favor of the codelab

Author: kwalrath

firebase.json

@@ -154,6 +154,7 @@
       { "source": "/linter/lints/:lint*", "destination": "https://dart-lang.github.io/linter/lints/:lints*", "type": 301 },
       { "source": "/mailing-list", "destination": "https://groups.google.com/a/dartlang.org/forum/#!forum/misc", "type": 301 },
       { "source": "/news{,/**}", "destination": "https://news.dartlang.org/", "type": 301 },
+      { "source": "/null-safety/tour", "destination": "/codelabs/null-safety", "type": 301 },
       { "source": "/observatory{,/**}", "destination": "https://dart-lang.github.io/observatory", "type": 301 },
       { "source": "/posters{,/**}", "destination": "https://github.com/dart-lang/www.dartlang.org/tree/f5f235e56e51d38d9f4d01f949d0a485f3828056/src/site/posters", "type": 301 },
       { "source": "/pub/tools/editor", "destination": "/tools", "type": 301 },

null_safety_examples/null_safety_codelab/bin/more_nullable_types.dart

@@ -1,9 +1,9 @@
 void main() {
-  List<String> aListofStrings = ['one', 'two', 'three'];
+  List<String> aListOfStrings = ['one', 'two', 'three'];
   List<String?> aNullableListOfStrings = [];
-  List<String?> aListofNullableStrings = ['one', null, 'three'];
+  List<String?> aListOfNullableStrings = ['one', null, 'three'];
 
-  print('aListofStrings is $aListofStrings.');
+  print('aListOfStrings is $aListOfStrings.');
   print('aNullableListOfStrings is $aNullableListOfStrings.');
-  print('aListofNullableStrings is $aListofNullableStrings.');
+  print('aListOfNullableStrings is $aListOfNullableStrings.');
 }

src/codelabs/index.md

@@ -31,6 +31,11 @@ with special attention to filtering and mapping values.
 Use DartPad to learn or remind yourself of how to write 
 asynchronous code using futures and the `async` and `await` keywords.
 
+### [Null safety](/codelabs/null-safety)
+
+Use DartPad to learn about Dart's null-safe type system,
+which was introduced in Dart 2.12.
+
 ## Flutter
 
 To learn about Flutter, try one of the

src/codelabs/null-safety.md

@@ -77,13 +77,13 @@ question marks to correct the type declarations of `aNullableListOfStrings` and
 <?code-excerpt "../null_safety_examples/null_safety_codelab/bin/more_nullable_types.dart" replace="/String\?/String/g"?>
 ```dart:run-dartpad:ga_id-nullable_type_generics:null_safety-true
 void main() {
-  List<String> aListofStrings = ['one', 'two', 'three'];
+  List<String> aListOfStrings = ['one', 'two', 'three'];
   List<String> aNullableListOfStrings = [];
-  List<String> aListofNullableStrings = ['one', null, 'three'];
+  List<String> aListOfNullableStrings = ['one', null, 'three'];
 
-  print('aListofStrings is $aListofStrings.');
+  print('aListOfStrings is $aListOfStrings.');
   print('aNullableListOfStrings is $aNullableListOfStrings.');
-  print('aListofNullableStrings is $aListofNullableStrings.');
+  print('aListOfNullableStrings is $aListOfNullableStrings.');
 }
 ```
 
@@ -322,3 +322,7 @@ are some suggestions for where to go next:
 
 If you're interested in using embedded DartPads, like this codelab does, see
 [best practices for using DartPad in tutorials](/resources/dartpad-best-practices).
+If you're interested in improving this codelab, see
+[issue #3093][].
+
+[issue #3093]: https://github.com/dart-lang/site-www/issues/3093

src/null-safety/index.md

@@ -38,9 +38,9 @@ or learn about null safety using
 
 ![Screenshot of DartPad in null-safe mode](/null-safety/dartpad-snippet.png)
 
-For an example-driven summary of null safety language features,
-see the [null safety feature tour](/null-safety/tour).
-For a more in-depth discussion, see
+For an interactive, example-driven introduction to null safety language features,
+see the [null safety codelab][Null safety codelab].
+For an in-depth discussion, see
 [Understanding null safety](/null-safety/understanding-null-safety).
 
 
@@ -131,7 +131,7 @@ The Dart team is currently aware of the following issues:
 
 For more information about null safety, see the following resources:
 
-* [Null safety tour][]
+* [Null safety codelab][]
 * [Understanding null safety][]
 * [Migration guide for existing code][migration guide]
 * [Null safety FAQ][]
@@ -146,7 +146,7 @@ For more information about null safety, see the following resources:
 [Dart blog]: https://medium.com/dartlang
 [migration guide]: /null-safety/migration-guide
 [Null safety FAQ]: /null-safety/faq
-[Null safety tour]: /null-safety/tour
+[Null safety codelab]: /codelabs/null-safety
 [nullsafety.dartpad.dev]: https://nullsafety.dartpad.dev
 [Understanding null safety]: /null-safety/understanding-null-safety