Fix for NPE when FieldFormalParameterElement references an unresolved field.

pkg/analysis_server/test/analysis/notification_navigation_test.dart

Index: pkg/analysis_server/test/analysis/notification_navigation_test.dart
diff --git a/pkg/analysis_server/test/analysis/notification_navigation_test.dart b/pkg/analysis_server/test/analysis/notification_navigation_test.dart
index 6b0148b2ba9c70ac4bb9a19353cef0b2f7452aaf..e64876317f95bded947e3587c9cf30ed89db269b 100644
--- a/pkg/analysis_server/test/analysis/notification_navigation_test.dart
+++ b/pkg/analysis_server/test/analysis/notification_navigation_test.dart
@@ -233,6 +233,17 @@ class AAA {
     });
   }
 
+  test_fieldFormalParameter_unresolved() {
+    addTestFile('''
+class AAA {
+  AAA(this.fff);
+}
+''');
+    return prepareNavigation().then((_) {
+      assertNoRegion('fff);', 3);
+    });
+  }
+
   test_identifier_resolved() {
     addTestFile('''
 class AAA {}