Fix VERBOSE_FLUTTER_COMPLETIONS flag.

pkg/analysis_server/lib/src/server/driver.dart

Index: pkg/analysis_server/lib/src/server/driver.dart
diff --git a/pkg/analysis_server/lib/src/server/driver.dart b/pkg/analysis_server/lib/src/server/driver.dart
index 497727e8a9634f2b7216f700fa53a8825babfa98..ca42c591aab627ae730210618839f7ab51c08d1d 100644
--- a/pkg/analysis_server/lib/src/server/driver.dart
+++ b/pkg/analysis_server/lib/src/server/driver.dart
@@ -561,7 +561,7 @@ class Driver implements ServerStarter {
         negatable: false);
     parser.addOption(NEW_ANALYSIS_DRIVER_LOG,
         help: "set a destination for the new analysis driver's log");
-    parser.addOption(VERBOSE_FLUTTER_COMPLETIONS,
+    parser.addFlag(VERBOSE_FLUTTER_COMPLETIONS,
         help: "enable verbose code completion for Flutter (experimental)");
     parser.addOption(PORT_OPTION,
         help: "the http diagnostic port on which the server provides"