Issue 23568. Fix for NPE in index when unresolved field formal parameter.

pkg/analysis_server/lib/src/services/index/indexable_element.dart

 // Copyright (c) 2015, 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 src.services.index;
 
 import 'dart:collection';
 
 import 'package:analysis_server/analysis/index/index_core.dart';
 import 'package:analyzer/src/generated/element.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/source.dart';
 
 /**
  * A wrapper around an [Element] that implements the [IndexableObject] interface.
  */
 class IndexableElement implements IndexableObject {
   /**
    * The element being wrapped.
    */
   final Element element;
 
   /**
    * Initialize a newly created wrapper to wrap the given [element].
    */
-  IndexableElement(this.element);
+  IndexableElement(this.element) {
+    if (element == null) {
+      throw new ArgumentError.notNull('element');
+    }
+  }
 
   @override
   int get hashCode => element.hashCode;
 
   @override
   IndexableObjectKind get kind => IndexableElementKind.forElement(element);
 
   @override
   int get length => element.displayName.length;
 
@@ skipping 108 matching lines @@
    * Return the indexable element kind representing the given [element].
    */
   static IndexableElementKind forElement(Element element) {
     if (element is ConstructorElement) {
       ClassElement classElement = element.enclosingElement;
       int constructorIndex = classElement.constructors.indexOf(element);
       return _constructorKinds.putIfAbsent(constructorIndex,
           () => new IndexableElementKind._(ElementKind.CONSTRUCTOR));
     }
     ElementKind elementKind = element.kind;
-    return _kindMap.putIfAbsent(elementKind, () =>
-        new IndexableElementKind._(elementKind));
+    return _kindMap.putIfAbsent(
+        elementKind, () => new IndexableElementKind._(elementKind));
   }
 }