Don't index constructor element reference second time.

pkg/analyzer/lib/src/summary/index_unit.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/src/dart/element/member.dart';
 import 'package:analyzer/src/generated/utilities_dart.dart';
@@ skipping 388 matching lines @@
     if (node.name != null) {
       int offset = node.period.offset;
       int length = node.name.end - offset;
       recordRelationOffset(
           element, IndexRelationKind.IS_REFERENCED_BY, offset, length, true);
     } else {
       int offset = node.type.end;
       recordRelationOffset(
           element, IndexRelationKind.IS_REFERENCED_BY, offset, 0, true);
     }
-    super.visitConstructorName(node);
+    node.type.accept(this);
   }
 
   @override
   visitExportDirective(ExportDirective node) {
     ExportElement element = node.element;
     recordUriReference(element?.exportedLibrary, node);
     super.visitExportDirective(node);
   }
 
   @override
@@ skipping 247 matching lines @@
         usedNames: nameRelations.map((r) => r.nameId).toList(),
         usedNameKinds: nameRelations.map((r) => r.kind).toList(),
         usedNameOffsets: nameRelations.map((r) => r.offset).toList());
   }
 
   void defineName(String name, IndexNameKind kind, int offset) {
     int nameId = pkg._getStringId(name);
     definedNames.add(new _DefinedNameInfo(nameId, kind, offset));
   }
 }