Fine. Move loadBundle() out of load(), tweaks for logging and measuring.

We use performance operations now for better instrumentation, and
detailed logging does not seem useful to me.

Change-Id: I87a16a8de37ec3696cc33f981007569ee67c491a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/443184
Reviewed-by: Johnni Winther <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>

Author: scheglov