Fix for exception duirng indexing a reference to MultiplyDefinedElement.

pkg/analyzer/test/src/dart/analysis/index_test.dart

Index: pkg/analyzer/test/src/dart/analysis/index_test.dart
diff --git a/pkg/analyzer/test/src/dart/analysis/index_test.dart b/pkg/analyzer/test/src/dart/analysis/index_test.dart
index 04048636e3235c44ebea7f5b9cf1798b6aea1137..d090dbb85232f681bb2ad9db6c8b2eaedf6f3ec3 100644
--- a/pkg/analyzer/test/src/dart/analysis/index_test.dart
+++ b/pkg/analyzer/test/src/dart/analysis/index_test.dart
@@ -793,6 +793,16 @@ class A {
       ..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}) {}