So the ecmascript package only works if you are running the Meteor command line tool from 1.2. So you would need to use that versionsFromand run meteor --release 1.2-rc.7.
This is kind of a shitty error though, I wonder if we could make it better.
OK just consulted with @glasser and apparently this is just an RC thing. When Meteor 1.2 actually comes out it will print a nicer message that explains that you need to be using 1.2 for ecmascript to work.
Literally working 8 hours a day to push it out right now, so as soon as humanly possible. (It’s in my interest too, building and testing releases isn’t the most fun part of this job)