This hasn't made it into the official docs yet, but I recently merged a pull request that allows you exactly that:
See https://github.com/dalekjs/dalek-browser-chrome/pull/8 for more details.
You can add the instructions to your Dalekfile.json
like so:
{
"browsers": [{
"chrome": {
"chromeOptions": {
"args": ["enable-experimental-web-platform-features", "js-flags=--harmony"]
}
}
}]
}