Flow analysis: fix handling of promotions in `when` clauses with shared case bodies.

Previously, if multiple switch cases shared a body, and at least one
of those cases promoted a match variable using a `when` clause, the
promotion would not be carried over to the merged variable seen in the
shared case body.  This was happening because promotions performed in
the `when` clause were applied to the promotion key associated with
the client's representation of the variable, whereas flow analysis was
computing the state of the merged variable based on the promotion keys
used internally while visiting the pattern (which don't include
promotions from the `when` clause).

With this change, flow analysis now computes the merged variable state
based on the promotion keys associated with the client's
representation of the variable components, so promotions from `when`
clauses take effect.

In the process, I've moved a lot of the tracking of these promotion
keys from the `TypeAnalyzer` class to the `_FlowAnalysisImpl` class.
This avoids the need to expose more flow analysis internals through
its public API.

Fixes #51399.

Bug: https://github.com/dart-lang/sdk/issues/51399
Change-Id: I8f1bb6e49ceb8441bb743c63af61c119df9041f2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/283441
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>