Use escope's native implicit global variable search code instead of our own.

That code was buggy when we first wrote linker; we reported the bugs and the
maintainer fixed it. Yay!
Delete unused JSAnalyze.findGlobalDottedRefs.

Because it is currently unused and was the only direct use of the 'estraverse'
module, it seems better to minimize the number of dependencies shipped with
Meteor. Admittedly, estraverse is still a dependency of escope, but if we
upgrade escope to a version that has a more specific version requirement for
estraverse that conflicts with our Npm.depends, we'll be shipping two copies for
no reason.