Add extension on Completer to simplify code.

Change-Id: Ida74cf78f5cedf2c4b3c806c7aaef92fa86f6d61
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/397960
Commit-Queue: Keerti Parthasarathy <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>

pkg/analysis_server/lib/src/lsp/lsp_analysis_server.dart

@@ -561,9 +561,7 @@ class LspAnalysisServer extends AnalysisServer {
         } else {
           showErrorMessageToUser('Unknown message type');
         }
-        if (completer != null && !completer.isCompleted) {
-          completer.complete();
-        }
+        completer?.setComplete();
       } on InconsistentAnalysisException {
         sendErrorResponse(
           message,
@@ -572,9 +570,7 @@ class LspAnalysisServer extends AnalysisServer {
             message: 'Document was modified before operation completed',
           ),
         );
-        if (completer != null && !completer.isCompleted) {
-          completer.complete();
-        }
+        completer?.setComplete();
       } catch (error, stackTrace) {
         var errorMessage =
             message is ResponseMessage
@@ -592,9 +588,7 @@ class LspAnalysisServer extends AnalysisServer {
           ),
         );
         logException(errorMessage, error, stackTrace);
-        if (completer != null && !completer.isCompleted) {
-          completer.complete();
-        }
+        completer?.setComplete();
       }
     }, socketError);
   }
@@ -1395,3 +1389,12 @@ class LspServerContextManagerCallbacks
     );
   }
 }
+
+extension on Completer<void> {
+  void setComplete() {
+    if (isCompleted) {
+      return;
+    }
+    complete();
+  }
+}