Add error codes to server protocol (#25958).

pkg/analysis_server/doc/api.html

Index: pkg/analysis_server/doc/api.html
diff --git a/pkg/analysis_server/doc/api.html b/pkg/analysis_server/doc/api.html
index fd1e1ef7b2e8e5597b659012bec18e168c83a796..5e715f1e040a3c357301dd775109f6d99955079c 100644
--- a/pkg/analysis_server/doc/api.html
+++ b/pkg/analysis_server/doc/api.html
@@ -2454,6 +2454,11 @@ dt.typeDefinition {
               the error. The field is omitted if there is no correction
               message associated with the error code.
             </p>
+          </dd><dt class="field"><b><i>code ( String )</i></b></dt><dd>
+            
+            <p>
+              The name, as a string, of the error code associated with this error.
+            </p>
           </dd><dt class="field"><b><i>hasFix ( <span style="color:#999999">optional</span> bool )</i></b></dt><dd>
             
             <p>