Add error codes to server protocol (#25958).

pkg/analysis_server/test/protocol_server_test.dart

Index: pkg/analysis_server/test/protocol_server_test.dart
diff --git a/pkg/analysis_server/test/protocol_server_test.dart b/pkg/analysis_server/test/protocol_server_test.dart
index 24d03a319156d8cf80b85048df92e397025becd9..3a164f049c272a4a6386d0d7f38f60e96c9744b1 100644
--- a/pkg/analysis_server/test/protocol_server_test.dart
+++ b/pkg/analysis_server/test/protocol_server_test.dart
@@ -69,6 +69,7 @@ class AnalysisErrorTest {
       },
       MESSAGE: 'my message',
       CORRECTION: 'my correction',
+      CODE: 'ambiguous_export',
       HAS_FIX: false
     });
   }
@@ -87,6 +88,7 @@ class AnalysisErrorTest {
         START_COLUMN: 2
       },
       MESSAGE: 'my message',
+      CODE: 'ambiguous_export',
       HAS_FIX: false
     });
   }
@@ -105,6 +107,7 @@ class AnalysisErrorTest {
         START_COLUMN: -1
       },
       MESSAGE: 'my message',
+      CODE: 'ambiguous_export',
       HAS_FIX: false
     });
   }