[CLI] Add help message about --timeline-recorder to dart run --help

TEST=manual testing, CI

Change-Id: I11746e106c44bcef202f6002e10aa6e29381addb
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/288582
Reviewed-by: Ben Konyi <bkonyi@google.com>
Commit-Queue: Derek Xu <derekx@google.com>

pkg/dartdev/lib/src/commands/run.dart

@@ -129,7 +129,19 @@ class RunCommand extends DartdevCommand {
         ..addFlag(
           'enable-asserts',
           help: 'Enable assert statements.',
-        );
+        )
+        ..addOption('timeline-recorder',
+            help: 'Selects the timeline recorder to use.\n'
+                'Valid recorders include: none, ring, endless, startup, '
+                'systrace, file, callback.\n'
+                'Defaults to ring.',
+            valueHelp: 'recorder');
+    } else {
+      argParser.addOption('timeline-recorder',
+          help: 'Selects the timeline recorder to use.\n'
+              'Valid recorders include: none, systrace, file, callback.\n'
+              'Defaults to none.',
+          valueHelp: 'recorder');
     }
     argParser.addOption(
       'verbosity',

pkg/dartdev/test/commands/run_test.dart

@@ -332,8 +332,8 @@ void main(List<String> args) => print("$b $args");
             '}');
 
     final result = await p.run([
-      '--timeline-recorder=file',
       'run',
+      '--timeline-recorder=file',
       '--timeline-streams=Dart',
       p.relativeFilePath
     ]);

runtime/bin/main_options.cc

@@ -417,6 +417,7 @@ bool Options::ProcessVMDebuggingOptions(const char* arg,
   V("--warn-on-pause-with-no-debugger", arg)                                   \
   V("--no-warn-on-pause-with-no-debugger", arg)                                \
   V("--timeline-streams", arg)                                                 \
+  V("--timeline-recorder", arg)                                                \
   V("--enable-experiment", arg)
   HANDLE_DARTDEV_VM_DEBUG_OPTIONS(IS_DEBUG_OPTION, arg);