Do not print --output_directory in reproduction commands

Change-Id: I19229506a1f8eb3c13dff84794b9bba53cc5841e
Reviewed-on: https://dart-review.googlesource.com/c/79562
Reviewed-by: Alexander Thomas <athom@google.com>
Commit-Queue: Martin Kustermann <kustermann@google.com>

Author: mkustermann

tools/testing/dart/options.dart

@@ -341,6 +341,7 @@ compiler.''',
     'exclude_suite',
     'firefox',
     'local_ip',
+    'output_directory',
     'progress',
     'report',
     'safari',