Fix for NPE when FieldFormalParameterElement references an unresolved field.

pkg/analysis_server/test/analysis/notification_navigation_test.dart

 // Copyright (c) 2014, 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.
 
 library test.analysis.notification.navigation;
 
 import 'dart:async';
 
 import 'package:analysis_server/src/constants.dart';
 import 'package:analysis_server/src/protocol.dart';
@@ skipping 215 matching lines @@
 class AAA {
   int fff = 123;
   AAA(this.fff);
 }
 ''');
     return prepareNavigation().then((_) {
       assertHasRegionTarget('fff);', 'fff = 123');
     });
   }
 
+  test_fieldFormalParameter_unresolved() {
+    addTestFile('''
+class AAA {
+  AAA(this.fff);
+}
+''');
+    return prepareNavigation().then((_) {
+      assertNoRegion('fff);', 3);
+    });
+  }
+
   test_identifier_resolved() {
     addTestFile('''
 class AAA {}
 main() {
   AAA aaa = null;
   print(aaa);
 }
 ''');
     return prepareNavigation().then((_) {
       assertHasRegionTarget('AAA aaa', 'AAA {}');
@@ skipping 241 matching lines @@
   test_type_void() {
     addTestFile('''
 void main() {
 }
 ''');
     return prepareNavigation().then((_) {
       assertNoRegionAt('void');
     });
   }
 }