Begin implementing flow analysis for patterns.

This CL adds support for flow analysis with variable patterns and
guards, and integrates it with if-case elements, if-case statements,
pattern variable declarations, switch expressions, and switch
statements.  It includes support for guards.

No other types of patterns are handled yet.

Bug: #50419
Change-Id: Iacad82b472cba0e2e670981847258e4046017576
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/274162
Reviewed-by: Konstantin Shcheglov <[email protected]>
Commit-Queue: Paul Berry <[email protected]>

Author: stereotype441