Check for no constructors before iterating (issue 28703)

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 1988 matching lines @@
    */
   ConstructorElementImpl.forNode(Identifier name) : super.forNode(name);
 
   /**
    * Initialize using the given serialized information.
    */
   ConstructorElementImpl.forSerialized(
       UnlinkedExecutable serializedExecutable, ClassElementImpl enclosingClass)
       : super.forSerialized(serializedExecutable, enclosingClass);
 
+  /**
+   * Return the constant initializers for this element, which will be empty if
+   * there are no initializers, or `null` if there was an error in the source.
+   */
   List<ConstructorInitializer> get constantInitializers {
     if (serializedExecutable != null && _constantInitializers == null) {
       _constantInitializers ??= serializedExecutable.constantInitializers
           .map((i) => _buildConstructorInitializer(i))
           .toList(growable: false);
     }
     return _constantInitializers;
   }
 
   void set constantInitializers(
@@ skipping 6620 matching lines @@
 
   @override
   void visitElement(Element element) {
     int offset = element.nameOffset;
     if (offset != -1) {
       map[offset] = element;
     }
     super.visitElement(element);
   }
 }