Don't index constructor element reference second time.

pkg/analyzer/test/src/summary/index_unit_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:convert';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/src/summary/format.dart';
 import 'package:analyzer/src/summary/idl.dart';
@@ skipping 522 matching lines @@
 class A = B with M;
 class B = A with M;
 main() {
   new A();
   new B();
 }
 ''');
     // No additional validation, but it should not fail with stack overflow.
   }
 
+  void test_isReferencedBy_ConstructorElement_namedOnlyWithDot() {
+    _indexTestUnit('''
+class A {
+  A.named() {}
+}
+main() {
+  new A.named();
+}
+''');
+    // has ".named()", but does not have "named()"
+    int offsetWithoutDot = findOffset('named();');
+    int offsetWithDot = findOffset('.named();');
+    expect(unitIndex.usedElementOffsets, isNot(contains(offsetWithoutDot)));
+    expect(unitIndex.usedElementOffsets, contains(offsetWithDot));
+  }
+
   void test_isReferencedBy_ConstructorElement_redirection() {
     _indexTestUnit('''
 class A {
   A() : this.bar(); // 1
   A.foo() : this(); // 2
   A.bar();
 }
 ''');
     ClassElement classA = findElement('A');
     ConstructorElement constA = classA.constructors[0];
@@ skipping 374 matching lines @@
   void isNotReferencedAt(String search, {int length}) {
     test._assertUsedName(name, IndexRelationKind.IS_REFERENCED_BY,
         test._expectedLocation(search, true, length: length), true);
   }
 
   void isReferencedAt(String search, {int length}) {
     test._assertUsedName(name, IndexRelationKind.IS_REFERENCED_BY,
         test._expectedLocation(search, true, length: length), false);
   }
 }