[kernel] Clone Block.fileEndOffset
[kernel] Clone Block.fileEndOffset

+ remove redundant clone of fileOffset.

Change-Id: Ie13e35a710e8496da4e93b858ee18b54dd0d551f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/170960
Reviewed-by: Jens Johansen <05223a3c6bdf76af6f84755eae231abeebcf4530@google.com>
Commit-Queue: Johnni Winther <0f64ef784d57ffb064b634040f9eeba0d33d0be4@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk