Issue 21238. Fix for NPE in occurrences computer when FieldFormalParameterElement references unreso…

pkg/analysis_server/lib/src/computer/computer_occurrences.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 computer.occurrences;
 
 import 'dart:collection';
 
 import 'package:analysis_server/src/protocol_server.dart' as protocol;
 import 'package:analyzer/src/generated/ast.dart';
@@ skipping 19 matching lines @@
     List<protocol.Occurrences> occurrences = <protocol.Occurrences>[];
     _elementsOffsets.forEach((engineElement, offsets) {
       var serverElement = protocol.newElement_fromEngine(engineElement);
       var length = engineElement.displayName.length;
       occurrences.add(new protocol.Occurrences(serverElement, offsets, length));
     });
     return occurrences;
   }
 
   void _addOccurrence(Element element, int offset) {
+    element = _canonicalizeElement(element);
     if (element == null || element == DynamicElementImpl.instance) {
       return;
     }
-    element = _canonicalizeElement(element);
     List<int> offsets = _elementsOffsets[element];
     if (offsets == null) {
       offsets = <int>[];
       _elementsOffsets[element] = offsets;
     }
     offsets.add(offset);
   }
 
   Element _canonicalizeElement(Element element) {
     if (element is FieldFormalParameterElement) {
@@ skipping 17 matching lines @@
 
   @override
   visitSimpleIdentifier(SimpleIdentifier node) {
     Element element = node.bestElement;
     if (element != null) {
       computer._addOccurrence(element, node.offset);
     }
     return super.visitSimpleIdentifier(node);
   }
 }