[cfe] Move body builder helpers to their own part

This helps navigate the already fairly long body_builder.dart file.

Change-Id: I091a8c63ed95893a37e21c863fd1a8b45137cc1d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/448200
Reviewed-by: Jens Johansen <[email protected]>
Commit-Queue: Johnni Winther <[email protected]>

Author: johnniwinther