Combine inference and choosing the most specific extension.

R=brianwilkerson@google.com, pquitslund@google.com

Change-Id: Idb69695611f777727c127aeb629e5b62e8112648
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/112220
Reviewed-by: Phil Quitslund <pquitslund@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>

Author: scheglov