Use empty placeholders for List(s) in elements.

Change-Id: I1d63fe018a0a78cfdf2e9e7ec91aefdc9cd91bb8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/183500
Commit-Queue: Konstantin Shcheglov <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>

Author: scheglov