Fix warning in element_handle.dart.

pkg/analyzer/lib/src/generated/element_handle.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 engine.element_handle;
 
 import 'ast.dart';
 import 'element.dart';
 import 'engine.dart';
 import 'java_core.dart';
@@ skipping 877 matching lines @@
 
   @override
   MethodDeclaration computeNode() => actualElement.computeNode();
 }
 
 /**
  * Instances of the class `ParameterElementHandle` implement a handle to a
  * `ParameterElement`.
  */
 class ParameterElementHandle extends VariableElementHandle
+    with ParameterElementMixin
     implements ParameterElement {
   /**
    * Initialize a newly created element handle to represent the given element.
    *
    * @param element the element being represented
    */
   ParameterElementHandle(ParameterElement element) : super(element);
 
   @override
   ParameterElement get actualElement => super.actualElement as ParameterElement;
@@ skipping 207 matching lines @@
 }
 
 /**
  * TODO(scheglov) invalid implementation
  */
 class WeakReference<T> {
   final T value;
   WeakReference(this.value);
   T get() => value;
 }