Add error codes to server protocol (#25958).

pkg/analysis_server/test/integration/protocol_matchers.dart

Index: pkg/analysis_server/test/integration/protocol_matchers.dart
diff --git a/pkg/analysis_server/test/integration/protocol_matchers.dart b/pkg/analysis_server/test/integration/protocol_matchers.dart
index 6266cb93ba8599fa5f4fa669394ecb1cbdb27146..487d6a7a8b5d6a5326ed4640546a48f1370576bc 100644
--- a/pkg/analysis_server/test/integration/protocol_matchers.dart
+++ b/pkg/analysis_server/test/integration/protocol_matchers.dart
@@ -1070,6 +1070,7 @@ final Matcher isAddContentOverlay = new LazyMatcher(() => new MatchesJsonObject(
  *   "location": Location
  *   "message": String
  *   "correction": optional String
+ *   "code": String
  *   "hasFix": optional bool
  * }
  */
@@ -1078,7 +1079,8 @@ final Matcher isAnalysisError = new LazyMatcher(() => new MatchesJsonObject(
     "severity": isAnalysisErrorSeverity,
     "type": isAnalysisErrorType,
     "location": isLocation,
-    "message": isString
+    "message": isString,
+    "code": isString
   }, optionalFields: {
     "correction": isString,
     "hasFix": isBool