Fix for exception duirng indexing a reference to MultiplyDefinedElement.

pkg/analyzer/test/src/dart/analysis/index_test.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 'dart:async';
 import 'dart:convert';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/src/dart/analysis/driver.dart';
@@ skipping 775 matching lines @@
     print(this.method); // q
     print(method); // nq
   }
 }''');
     MethodElement element = findElement('method');
     assertThat(element)
       ..isReferencedAt('method); // q', true)
       ..isReferencedAt('method); // nq', false);
   }
 
+  test_isReferencedBy_MultiplyDefinedElement() async {
+    provider.newFile(_p('$testProject/a1.dart'), 'class A {}');
+    provider.newFile(_p('$testProject/a2.dart'), 'class A {}');
+    await _indexTestUnit('''
+import 'a1.dart';
+import 'a2.dart';
+A v = null;
+''');
+  }
+
   test_isReferencedBy_ParameterElement() async {
     await _indexTestUnit('''
 foo({var p}) {}
 main() {
   foo(p: 1);
 }
 ''');
     Element element = findElement('p');
     assertThat(element)..isReferencedAt('p: 1', true);
   }
@@ skipping 415 matching lines @@
   final bool isQualified;
 
   _Relation(this.kind, this.offset, this.length, this.isQualified);
 
   @override
   String toString() {
     return '_Relation{kind: $kind, offset: $offset, length: $length, '
         'isQualified: $isQualified}lified)';
   }
 }