Use null aware operators to clean up code

pkg/analysis_server/lib/src/services/correction/namespace.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 services.src.correction.namespace;
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/src/dart/resolver/scope.dart';
 
@@ skipping 28 matching lines @@
 /**
  * Returns the [ImportElement] that is referenced by [prefixNode] with
  * an [PrefixElement], maybe `null`.
  */
 ImportElement getImportElement(SimpleIdentifier prefixNode) {
   if (prefixNode.parent is ImportDirective) {
     ImportDirective importDirective = prefixNode.parent;
     return importDirective.element;
   }
   ImportElementInfo info = internal_getImportElementInfo(prefixNode);
-  return info != null ? info.element : null;
+  return info?.element;
 }
 
 /**
  * Return the [ImportElement] that declared [prefix] and imports [element].
  *
  * [libraryElement] - the [LibraryElement] where reference is.
  * [prefix] - the import prefix, maybe `null`.
  * [element] - the referenced element.
  * [importElementsMap] - the cache of [Element]s imported by [ImportElement]s.
  */
@@ skipping 113 matching lines @@
 }
 
 /**
  * Information about [ImportElement] and place where it is referenced using
  * [PrefixElement].
  */
 class ImportElementInfo {
   ImportElement element;
   int periodEnd;
 }