Use null-aware operators to clean up the code

pkg/analyzer/lib/src/dart/element/element.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.
 
 library analyzer.src.dart.element.element;
 
 import 'dart:collection';
 import 'dart:math' show min;
 
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 148 matching lines @@
   List<InterfaceType> get allSupertypes {
     List<InterfaceType> list = new List<InterfaceType>();
     _collectAllSupertypes(list);
     return list;
   }
 
   @override
   List<ConstructorElement> get constructors {
     if (!isMixinApplication) {
       assert(_constructors != null);
-      return _constructors == null
-          ? ConstructorElement.EMPTY_LIST
-          : _constructors;
+      return _constructors ?? ConstructorElement.EMPTY_LIST;
     }
-
     return _computeMixinAppConstructors();
   }
 
   /**
    * Set the constructors contained in this class to the given [constructors].
    *
    * Should only be used for class elements that are not mixin applications.
    */
   void set constructors(List<ConstructorElement> constructors) {
     assert(!isMixinApplication);
@@ skipping 4856 matching lines @@
 
   @override
   void visitElement(Element element) {
     int offset = element.nameOffset;
     if (offset != -1) {
       map[offset] = element;
     }
     super.visitElement(element);
   }
 }