Restrict analyzer/front_end/kernel SDK versions to dev.48+.

Change-Id: If341e8aa3503881bb44e445c39e5f821fd213bfd
Reviewed-on: https://dart-review.googlesource.com/55000
Reviewed-by: Kevin Moore <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>
Commit-Queue: Janice Collins <[email protected]>

Prepare for analyzer-0.31.2-alpha.2

Change-Id: Ib428b7316633bb77978428a17956bc05e736d57d
Reviewed-on: https://dart-review.googlesource.com/52981
Commit-Queue: Janice Collins <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>
Reviewed-by: Konstantin Shcheglov <[email protected]>
Reviewed-by: Dan Rubel <[email protected]>