Convert the summary request to use the elememnt model

Converting the summary request to use elements allows us to include API
from parts as well as API exported from other libraries in a uniform way.

Change-Id: Iad4982efc76c41331e0507bd6fc95e8ee0e4c9c6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/441833
Commit-Queue: Brian Wilkerson <[email protected]>
Reviewed-by: Jake Macdonald <[email protected]>

Author: bwilkerson