We need to use it in the observable package, which is externally
available, so it needs to be able to access it from pub.
Change-Id: I85a1488decb5ef4acd062e2fb3556e7d589e2c8c
Reviewed-on: https://dart-review.googlesource.com/44385
Reviewed-by: Leaf Petersen <leafp@google.com>
Commit-Queue: Bob Nystrom <rnystrom@google.com>