add null-aware operators to the language tour

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

 ---
 layout: book
 title: "Preface"
 subsite: "Dart Up and Running"
 description: "Read the preface of Dart: Up and Running (published by O'Reilly)."
 prev-chapter: foreword.html
 prev-chapter-title: "Foreword"
 next-chapter: ch01.html
 next-chapter-title: "Quick Start"
 ---
@@ skipping 88 matching lines @@
 
 This section gives details about how the book has changed between each
 of its printings, and how the online content has changed after the most
 recent printing.
 
 
 ### Changes after the third printing
 
 Added documentation for language features introduced after 1.0:
 
+- Null-aware operators:
+  - [??](ch02.html#conditional-expressions)
+  - [??=](ch02.html#assignment-operators)
+  - [?.](ch02.html#other-operators)
 - [Enumerated types](ch02.html#enums)
 - [Deferred loading](ch02.html#deferred-loading)
 - [Asynchrony support](ch02.html#asynchrony)
 
 Updated the [Keywords](ch02.html#keywords) section
 to add the following words,
 which are associated with new or proposed features:
 
 - async
 - async*
@@ skipping 360 matching lines @@
 who manage the author workflow and make working on an O’Reilly book such
 a pleasure. We personally worked with Sarah Schneider, Jessica Hosman,
 and Rachel James.
 
 Finally, we thank Lars Bak and Kasper Lund for writing the foreword, and
 most of all for creating Dart.
 
 
 <hr>
 {% include book-nav.html %}