Issue 23068. Constructors should have the same const/factory modifiers to match.

pkg/analyzer/lib/src/generated/incremental_resolver.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 engine.incremental_resolver;
 
 import 'dart:collection';
 import 'dart:math' as math;
 
 import 'package:analyzer/src/generated/constant.dart';
@@ skipping 167 matching lines @@
   }
 
   @override
   visitConstructorDeclaration(ConstructorDeclaration node) {
     _hasConstructor = true;
     SimpleIdentifier constructorName = node.name;
     ConstructorElementImpl element = constructorName == null
         ? _enclosingClass.unnamedConstructor
         : _enclosingClass.getNamedConstructor(constructorName.name);
     _processElement(element);
+    _assertEquals(node.constKeyword != null, element.isConst);
+    _assertEquals(node.factoryKeyword != null, element.isFactory);
     _assertCompatibleParameters(node.parameters, element.parameters);
     // TODO(scheglov) debug null Location
     if (element != null) {
       if (element.context == null || element.source == null) {
         logger.log(
             'Bad constructor element $element for $node in ${node.parent}');
       }
     }
     // matches, update the existing element
     ExecutableElement newElement = node.element;
@@ skipping 1642 matching lines @@
   @override
   String toString() => name;
 }
 
 class _TokenPair {
   final _TokenDifferenceKind kind;
   final Token oldToken;
   final Token newToken;
   _TokenPair(this.kind, this.oldToken, this.newToken);
 }