includeExternalHelpers breaks parallel builds

[boris-petrov]

Steps to reproduce: ember new foo, edit ember-cli-build.js, add includeExternalHelpers: true in ember-cli-babel, run THROW_UNLESS_PARALLELIZABLE=1 ember serve. You'll get:

[Babel: Babel: foo] was configured to `throwUnlessParallelizable` and was unable to parallelize a plugin.
plugins:
1: name: unknown, location: unknown