Use null-aware operators to clean up the code

pkg/analyzer/lib/src/dart/resolver/scope.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.resolver.scope;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/element.dart';
@@ skipping 306 matching lines @@
    */
   LabelScope(this._outerScope, this._label, this.node, this.element);
 
   /**
    * Return the LabelScope which defines [targetLabel], or `null` if it is not
    * defined in this scope.
    */
   LabelScope lookup(String targetLabel) {
     if (_label == targetLabel) {
       return this;
-    } else if (_outerScope != null) {
-      return _outerScope.lookup(targetLabel);
-    } else {
-      return null;
     }
+    return _outerScope?.lookup(targetLabel);
   }
 }
 
 /**
  * The scope containing all of the names available from imported libraries.
  */
 class LibraryImportScope extends Scope {
   /**
    * The element representing the library in which this scope is enclosed.
    */
@@ skipping 861 matching lines @@
 
   /**
    * Define the type parameters declared by the [classElement].
    */
   void _defineTypeParameters(ClassElement classElement) {
     for (TypeParameterElement typeParameter in classElement.typeParameters) {
       define(typeParameter);
     }
   }
 }