Use null-aware operators to clean up the code

pkg/analyzer/lib/dart/ast/ast.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /**
  * Defines the AST model. The AST (Abstract Syntax Tree) model describes the
  * syntactic (as opposed to semantic) structure of Dart code. The semantic
  * structure of the code is modeled by the
  * [element model](../element/element.dart).
  *
@@ skipping 4449 matching lines @@
         if (compare != 0) {
           return compare;
         }
       }
     }
     //
     // as
     //
     SimpleIdentifier prefix1 = import1.prefix;
     SimpleIdentifier prefix2 = import2.prefix;
-    String prefixStr1 = prefix1 != null ? prefix1.name : null;
-    String prefixStr2 = prefix2 != null ? prefix2.name : null;
+    String prefixStr1 = prefix1?.name;
+    String prefixStr2 = prefix2?.name;
     if (prefixStr1 != null || prefixStr2 != null) {
       if (prefixStr1 == null) {
         return -1;
       } else if (prefixStr2 == null) {
         return 1;
       } else {
         int compare = prefixStr1.compareTo(prefixStr2);
         if (compare != 0) {
           return compare;
         }
@@ skipping 3623 matching lines @@
   /**
    * Return the 'yield' keyword.
    */
   Token get yieldKeyword;
 
   /**
    * Return the 'yield' keyword to the given [token].
    */
   void set yieldKeyword(Token token);
 }