expired experimental flag command line message

Change-Id: I02a977f36969aec7fa67593739330fd785fcf02c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/107661
Commit-Queue: Johnni Winther <[email protected]>
Auto-Submit: Dan Rubel <[email protected]>
Reviewed-by: Johnni Winther <[email protected]>

Author: danrubel

pkg/front_end/lib/src/api_prototype/compiler_options.dart

@@ -233,6 +233,8 @@ Map<ExperimentalFlag, bool> parseExperimentalFlags(
       ExperimentalFlag flag = parseExperimentalFlag(experiment);
       if (flag == null) {
         onError("Unknown experiment: " + experiment);
+      } else if (flag == ExperimentalFlag.expiredFlag) {
+        print("Experiment flag no longer required: " + experiment);
       } else if (flags.containsKey(flag)) {
         if (flags[flag] != value) {
           onError(