Don't attempt to record DynamicElementImpl references.

pkg/analyzer/test/src/summary/index_unit_test.dart

Index: pkg/analyzer/test/src/summary/index_unit_test.dart
diff --git a/pkg/analyzer/test/src/summary/index_unit_test.dart b/pkg/analyzer/test/src/summary/index_unit_test.dart
index 3a1c4448d82c18600d38d170bce04fc5f5092607..a8a3f4f042604c99bfb629718e1fa6c46d160c48 100644
--- a/pkg/analyzer/test/src/summary/index_unit_test.dart
+++ b/pkg/analyzer/test/src/summary/index_unit_test.dart
@@ -671,6 +671,13 @@ main() {
     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 {