As of Meteor 0.9.0, Meteor supports packages with OS-specific builds (such as
packages that use binary npm modules). This allows you to deploy apps from one
platform to another, as long as the package has already published to the package
server in the appropriate target architecture.

However, this only works if the package can be loaded from the server at all; it
does not work if the package is only available as local source code in your app,
or in the main meteor tree if you're running meteor from a git checkout.

Specifically, this means that packages *in the Meteor core* which have
OS-specific dependencies will prevent apps from being deployed, if meteor is
being run from the git checkout. To simplify this, we're taking the OS-specific
dependencies of core packages and moving them to these packages, which are *NOT*
in "the core". In order to run meteor from a checkout, these packages must be
already published to the server.  Fortunately, they won't change very
frequently.