Fix for NPE when FieldFormalParameterElement references an unresolved field.

pkg/analysis_server/lib/src/computer/computer_navigation.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.navigation;
 
 import 'package:analysis_server/src/protocol.dart' as protocol;
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/element.dart';
 import 'package:analyzer/src/generated/scanner.dart';
@@ skipping 12 matching lines @@
 
   /**
    * Returns the computed navigation regions, not `null`.
    */
   List<protocol.NavigationRegion> compute() {
     _unit.accept(new _DartUnitNavigationComputerVisitor(this));
     return new List.from(_regions);
   }
 
   void _addRegion(int offset, int length, Element element) {
+    if (element is FieldFormalParameterElement) {
+      element = (element as FieldFormalParameterElement).field;
+    }
     if (element == null || element == DynamicElementImpl.instance) {
       return;
     }
-    if (element is FieldFormalParameterElement) {
-      element = (element as FieldFormalParameterElement).field;
-    }
     protocol.Element target = new protocol.Element.fromEngine(element);
     _regions.add(new protocol.NavigationRegion(offset, length, [target]));
   }
 
   void _addRegionForNode(AstNode node, Element element) {
     int offset = node.offset;
     int length = node.length;
     _addRegion(offset, length, element);
   }
 
@@ skipping 149 matching lines @@
 
   @override
   visitSimpleIdentifier(SimpleIdentifier node) {
     if (node.parent is ConstructorDeclaration) {
     } else {
       computer._addRegionForNode(node, node.bestElement);
     }
     return super.visitSimpleIdentifier(node);
   }
 }