add null-aware operators to the language tour

src/site/docs/dart-up-and-running/ch03.markdown

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 layout: book
 title: "A Tour of the Dart Libraries"
 subsite: "Dart Up and Running"
 description: "Read Chapter 3, A Tour of the Dart Libraries (from Dart: Up and Running, published by O'Reilly)."
 prev-chapter: ch02.html
 prev-chapter-title: "Language Tour"
 next-chapter: ch04.html
 next-chapter-title: "Tools"
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@@ skipping 949 matching lines @@
 
 Throwing an application-specific exception is a common way to indicate
 that an error has occurred. You can define a custom exception by
 implementing the Exception interface:
 
 <!-- ch03/exceptions.dart -->
 {% prettify dart %}
 class FooException implements Exception {
   final String msg;
   const FooException([this.msg]);
-  String toString() => msg == null ? 'FooException' : msg;
+  String toString() => msg ?? 'FooException';
 }
 {% endprettify %}
 
 For more information, see [Exceptions](#exceptions) and the [Exception API
 docs.](http://api.dartlang.org/dart_core/Exception.html)
 
 
 ## dart:async - asynchronous programming {#dartasync---asynchronous-programming}
 
 Asynchronous programming often uses callback functions, but Dart
@@ skipping 1815 matching lines @@
 can get yet more libraries by using the pub tool, discussed in the next
 chapter. The [args,](https://pub.dartlang.org/packages/args)
 [logging,](https://pub.dartlang.org/packages/logging)
 [polymer,](https://pub.dartlang.org/packages/polymer) and
 [test](https://pub.dartlang.org/packages/test) libraries are just a
 sampling of what you can install using pub.
 
 
 <hr>
 {% include book-nav.html %}