BTW, knowing virtually zero polymer, I did a little grep'ing to see how easily I could add this option myself and noticed the button's logic is implemented in two different places:
https://chromium.googlesource.com/infra/luci/luci-py/+/95f1b5af847bc3b85d9a402a6bf05d191d9b95ba/appengine/swarming/ui/res/imp/taskpage/task-page.html#1140
https://chromium.googlesource.com/infra/luci/luci-py/+/95f1b5af847bc3b85d9a402a6bf05d191d9b95ba/appengine/swarming/ui/build/elements.html

Is that WAI? Can one be removed?

It's tricky to do in JS, as it means two things:
- Extracting the isolated
- Fetch the command from it and be smart about it, e.g. vs raw command.

One thing I'd like to see happen is *turning down support for command in .isolated files*. One way to start working on that is to work on issue 665183.

Your observation in #1 is WAI - the latter is a "built" version of the former (and the rest of the elements).  The latter is what is served on the bots.

Re #2: That sounds understandably non-trivial. Feel free to close this as wontfix then. Though, maybe we can print a line like "See the command listed in isolateserver.appspot.com/browse?hash=the_isolate_hash to rerun task"?

I'm fine with this hack, it'd be relatively easy to code and fairly safe, pseudo-code:

if properties.command:
  print properties.command
elif properties.isolated:
  print `See https://isolate/browser...` and add properties.extra_args

Not awesome, still less bad and not hard to implement.

When we do this we should also set env and env_prefixes.

Maybe the best way to do this would be to have the bot prepare some helper script in the root of the task that could be `eval'd` (or the windows equivalent .bat file) that sets up the envvars and also prints the command that would have run?