Elements. Renames for builders, fragments, some simplifications.

Change-Id: I67103db05cc9beb4a573f4bfc480532c55265a86
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/392060
Reviewed-by: Brian Wilkerson <[email protected]>
Reviewed-by: Phil Quitslund <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>

Author: scheglov