Add entension support to the summary idl

Change-Id: Ic089352b9fe9007220c66b7ce389302dd2a09c59
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/108200
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>

Author: bwilkerson