Integration test search.getTypeHierarchy and make some minor fixes.

pkg/analysis_server/lib/src/protocol.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 protocol;
 
 import 'dart:collection';
 import 'dart:convert' show JsonDecoder;
 
 import 'package:analysis_services/json.dart';
@@ skipping 483 matching lines @@
    */
   final String id;
 
   /**
    * The error that was caused by attempting to handle the request, or `null` if
    * there was no error.
    */
   final RequestError error;
 
   /**
-   * A table mapping the names of result fields to their values. The table
-   * should be empty if there was an error.
+   * A table mapping the names of result fields to their values.  Should be
+   * null if there is no result to send.
    */
-  final Map<String, Object> result = new HashMap<String, Object>();
+  Map<String, Object> result;
 
   /**
    * Initialize a newly created instance to represent a response to a request
    * with the given [id]. If an [error] is provided then the response will
    * represent an error condition.
    */
   Response(this.id, [this.error]);
 
   /**
    * Initialize a newly created instance to represent an error condition caused
@@ skipping 106 matching lines @@
       return response;
     } catch (exception) {
       return null;
     }
   }
 
   /**
    * Return the value of the result field with the given [name].
    */
   Object getResult(String name) {
-    return result[name];
+    return result == null ? null : result[name];
   }
 
   /**
    * Set the value of the result field with the given [name] to the given [value].
    */
   void setResult(String name, Object value) {
+    if (result == null) {
+      result = new HashMap<String, Object>();
+    }
     result[name] = _toJson(value);
   }
 
   /**
+   * Set the result to be an empty map.
+   */
+  void setEmptyResult() {
+    result = new HashMap<String, Object>();
+  }
+
+  /**
    * Return a table representing the structure of the Json object that will be
    * sent to the client to represent this response.
    */
   Map<String, Object> toJson() {
     Map<String, Object> jsonObject = new HashMap<String, Object>();
     jsonObject[ID] = id;
     if (error != null) {
       jsonObject[ERROR] = error.toJson();
     }
-    if (!result.isEmpty) {
+    if (result != null) {
       jsonObject[RESULT] = result;
     }
     return jsonObject;
   }
 }
 
 /**
  * Instances of the class [RequestError] represent information about an error
  * that occurred while attempting to respond to a [Request].
  */
@@ skipping 280 matching lines @@
  */
 _toJson(Object value) {
   if (value is HasToJson) {
     return value.toJson();
   }
   if (value is Iterable) {
     return value.map((item) => _toJson(item)).toList();
   }
   return value;
 }