Shared type analysis: add optional exhaustiveness error.

This optional error fires if patterns are enabled and the type
analyzer encounters a switch that is required to be exhaustive (i.e. a
switch expression, or a switch statement with an always-exhaustive
type), and flow analysis cannot prove that the switch is exhaustive.

This is intended to be available as a temporary workaround if we
decide to ship an early beta of the "patterns" feature before
exhaustiveness checking is sufficiently ready.  We won't enable it
unless we need to, and we won't ship the final patterns feature with
it enabled.

Change-Id: I8c4c65c21abdc32b91537b3f30e7fd86f446f571
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/283060
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>