Add an interface for request parameters

pkg/analyzer_plugin/lib/src/protocol/protocol_internal.dart

 // Copyright (c) 2017, 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.
 
 import 'dart:collection';
 import 'dart:convert' hide JsonDecoder;
 
 import 'package:analyzer_plugin/protocol/generated_protocol.dart';
 import 'package:analyzer_plugin/protocol/protocol.dart';
 
@@ skipping 453 matching lines @@
         _request, jsonPath, buffer.toString()));
   }
 
   @override
   dynamic missingKey(String jsonPath, String key) {
     return new RequestFailure(RequestErrorFactory.invalidParameter(
         _request, jsonPath, 'Expected to contain key ${JSON.encode(key)}'));
   }
 }
 
+abstract class RequestParams implements HasToJson {
+  /**
+   * Return a request whose parameters are taken from this object and that has
+   * the given [id].
+   */
+  Request toRequest(String id);
+}
+
 /**
  * JsonDecoder for decoding responses from the server.  This is intended to be
  * used only for testing.  Errors are reported using bare [Exception] objects.
  */
 class ResponseDecoder extends JsonDecoder {
   @override
   final RefactoringKind refactoringKind;
 
   ResponseDecoder(this.refactoringKind);
 
@@ skipping 21 matching lines @@
 /**
  * The result data associated with a response.
  */
 abstract class ResponseResult implements HasToJson {
   /**
    * Return a response whose result data is this object for the request with the
    * given [id].
    */
   Response toResponse(String id);
 }