Fix for exception duirng indexing a reference to MultiplyDefinedElement.

pkg/analyzer/lib/src/dart/analysis/index.dart

 // Copyright (c) 2016, 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.
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
 import 'package:analyzer/src/dart/element/member.dart';
@@ skipping 408 matching lines @@
    * [offset] and [length].  The flag [isQualified] is `true` if the relation
    * has an explicit or implicit qualifier, so [element] cannot be shadowed by
    * a local declaration.
    */
   void recordRelationOffset(Element element, IndexRelationKind kind, int offset,
       int length, bool isQualified) {
     // Ignore elements that can't be referenced outside of the unit.
     ElementKind elementKind = element?.kind;
     if (elementKind == null ||
         elementKind == ElementKind.DYNAMIC ||
+        elementKind == ElementKind.ERROR ||
         elementKind == ElementKind.LABEL ||
         elementKind == ElementKind.LOCAL_VARIABLE ||
         elementKind == ElementKind.PREFIX ||
         elementKind == ElementKind.TYPE_PARAMETER ||
         elementKind == ElementKind.FUNCTION &&
             element is FunctionElement &&
             element.enclosingElement is ExecutableElement ||
         elementKind == ElementKind.PARAMETER &&
             element is ParameterElement &&
             element.parameterKind != ParameterKind.NAMED ||
@@ skipping 352 matching lines @@
   final String value;
 
   /**
    * The unique id of the string.  It is set after indexing of the whole
    * package is done and we are assembling the full package index.
    */
   int id;
 
   _StringInfo(this.value);
 }