Don't attempt to record DynamicElementImpl references.

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 653 matching lines @@
 main() {
   new A(); // 1
 }
 ''');
     ClassElement classA = findElement('A');
     ConstructorElement constA = classA.constructors[0];
     // A()
     assertThat(constA)..isReferencedAt('(); // 1', true, length: 0);
   }
 
+  void test_isReferencedBy_DynamicElement() {
+    _indexTestUnit('''
+dynamic f() {
+}''');
+    expect(unitIndex.usedElementOffsets, isEmpty);
+  }
+
   void test_isReferencedBy_FieldElement() {
     _indexTestUnit('''
 class A {
   var field;
   A({this.field});
   m() {
     field = 2; // nq
     print(field); // nq
   }
 }
@@ skipping 531 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}';
   }
 }