Fix for NPE in navigation after some incremental changes.

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 'dart:collection';
 
 import 'package:analysis_server/src/protocol_server.dart' as protocol;
 import 'package:analyzer/src/generated/ast.dart';
@@ skipping 26 matching lines @@
     if (element is FieldFormalParameterElement) {
       element = (element as FieldFormalParameterElement).field;
     }
     if (element == null || element == DynamicElementImpl.instance) {
       return;
     }
     if (element.location == null) {
       return;
     }
     int targetIndex = _addTarget(element);
+    // TODO(scheglov) Fix the enclosing element problem in the incremental
+    // resolver and remove this.
+    if (targetIndex == null) {
+      return;
+    }
     regions.add(
         new protocol.NavigationRegion(offset, length, <int>[targetIndex]));
   }
 
   int _addTarget(Element element) {
     int index = targetMap[element];
     if (index == null) {
       index = targets.length;
       protocol.NavigationTarget target =
           protocol.newNavigationTarget_fromElement(element, _addFile);
+      // TODO(scheglov) Fix the enclosing element problem in the incremental
+      // resolver and remove this.
+      if (target == null) {
+        return null;
+      }
       targets.add(target);
       targetMap[element] = index;
     }
     return index;
   }
 
   int _addFile(String file) {
     int index = fileMap[file];
     if (index == null) {
       index = files.length;
@@ skipping 177 matching lines @@
     Element element = node.bestElement;
     computer._addRegionForNode(node, element);
   }
 
   void _safelyVisit(AstNode node) {
     if (node != null) {
       node.accept(this);
     }
   }
 }