Make analyzer more strong mode compliant

pkg/analyzer/lib/src/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.
 
 library analyzer.src.dart.ast.ast;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
@@ skipping 3373 matching lines @@
  *      | [ImportDirective]
  *      | [LibraryDirective]
  *      | [PartDirective]
  *      | [PartOfDirective]
  */
 abstract class DirectiveImpl extends AnnotatedNodeImpl implements Directive {
   /**
    * The element associated with this directive, or `null` if the AST structure
    * has not been resolved or if this directive could not be resolved.
    */
-  @override
-  Element element;
+  Element _element;
 
   /**
    * Initialize a newly create directive. Either or both of the [comment] and
    * [metadata] can be `null` if the directive does not have the corresponding
    * attribute.
    */
   DirectiveImpl(Comment comment, List<Annotation> metadata)
       : super(comment, metadata);
+
+  @override
+  Element get element => _element;
+
+  /**
+   * Set the element associated with this directive to be the given [element].
+   */
+  void set element(Element element) {
+    _element = element;
+  }
 }
 
 /**
  * A do statement.
  *
  *    doStatement ::=
  *        'do' [Statement] 'while' '(' [Expression] ')' ';'
  */
 class DoStatementImpl extends StatementImpl implements DoStatement {
   /**
@@ skipping 7588 matching lines @@
 
   @override
   dynamic/*=E*/ accept/*<E>*/(AstVisitor/*<E>*/ visitor) =>
       visitor.visitYieldStatement(this);
 
   @override
   void visitChildren(AstVisitor visitor) {
     _expression?.accept(visitor);
   }
 }