Use null-aware operators to clean up the code

pkg/analyzer/lib/src/dart/resolver/inheritance_manager.dart

 // Copyright (c) 2016, 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.
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
@@ skipping 443 matching lines @@
    * @param classElt the class element to query
    * @param visitedInterfaces a set of visited classes passed back into this method when it calls
    *          itself recursively
    * @return `null` if there was a problem (such as a loop in the class hierarchy) or if there
    *         are no classes above this one in the class hierarchy. Otherwise, a list of interface
    *         lookup maps.
    */
   List<Map<String, ExecutableElement>> _gatherInterfaceLookupMaps(
       ClassElement classElt, HashSet<ClassElement> visitedInterfaces) {
     InterfaceType supertype = classElt.supertype;
-    ClassElement superclassElement =
-        supertype != null ? supertype.element : null;
+    ClassElement superclassElement = supertype?.element;
     List<InterfaceType> mixins = classElt.mixins;
     List<InterfaceType> interfaces = classElt.interfaces;
     // Recursively collect the list of mappings from all of the interface types
     List<Map<String, ExecutableElement>> lookupMaps =
         new List<Map<String, ExecutableElement>>();
     //
     // Superclass element
     //
     if (superclassElement != null) {
       if (!visitedInterfaces.contains(superclassElement)) {
@@ skipping 754 matching lines @@
   }
 
   /**
    * Initializes [keys] and [values].
    */
   void _initArrays(int initialCapacity) {
     _keys = new List<String>(initialCapacity);
     _values = new List<ExecutableElement>(initialCapacity);
   }
 }