Add error codes to server protocol (#25958).

pkg/analysis_server/test/integration/protocol_matchers.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.
 //
 // This file has been automatically generated.  Please do not edit it manually.
 // To regenerate the file, use the script
 // "pkg/analysis_server/tool/spec/generate_files".
 
 /**
  * Matchers for data types defined in the analysis server API
@@ skipping 1052 matching lines @@
 
 /**
  * AnalysisError
  *
  * {
  *   "severity": AnalysisErrorSeverity
  *   "type": AnalysisErrorType
  *   "location": Location
  *   "message": String
  *   "correction": optional String
+ *   "code": String
  *   "hasFix": optional bool
  * }
  */
 final Matcher isAnalysisError = new LazyMatcher(() => new MatchesJsonObject(
   "AnalysisError", {
     "severity": isAnalysisErrorSeverity,
     "type": isAnalysisErrorType,
     "location": isLocation,
-    "message": isString
+    "message": isString,
+    "code": isString
   }, optionalFields: {
     "correction": isString,
     "hasFix": isBool
   }));
 
 /**
  * AnalysisErrorFixes
  *
  * {
  *   "error": AnalysisError
@@ skipping 1383 matching lines @@
  *
  * {
  *   "newName": String
  * }
  */
 final Matcher isRenameOptions = new LazyMatcher(() => new MatchesJsonObject(
   "rename options", {
     "newName": isString
   }));