I don't think we can reasonably do this. I spent some time looking at different options, but it's pretty painful.

One option would be to allow
    foo(num, format) => Intl.plural(foo, other: "${format.format(num)} left", format: format);
but that's fairly ugly in terms of parsing. We only allow simple references to arguments, except for something that is of the form {<name_of_the_format_argument>.format(<name_of_the_howMany_argument>} and we need to test to see if a format argument was passed. But multiple source messages with the same text get combined to one translation. So if a format is passed in one instance but not another, do we treat that as making them different? Or as an error? Finding out if a format was passed is also very painful. We can have plurals nested inside genders or selects, multiple plurals (discouraged, but not disallowed), and when we parse it back from the ARB file the format is lost so we'd have dive into the internals of the original message as parsed from the source.

Alternatively we could make the breaking change that we always have a format, and if one is not passed we default to new NumberFormat(). But we still have the problem that the parsed input doesn't tell us which variable is the format (and there could be more than one if there are multiple plurals). While the alternative is awkward, something like

foo(num, format) => foo2(num, format.format(num));
foo2(num, formatted) => Intl.plural(num, other: "$formatted left"...);

the cure is seeming worse.

Added WontFix label.

This issue has been moved to dart-lang/i18n#310.