To be able to publish a package that uses ecmascript (or any other new packages), you should create a dummy project with the PLUGINS-PREVIEW@2 release and work on it as a local package within that. The reason for this is that the Meteor version in a plain package directory (not within an app) defaults to the system-level version.
Then to use that package in other apps, you’ll have to have updated those apps to to that Meteor release as well.