[stable] Flow analysis: fix first phase handling of pattern assignments.

Prior to this change, the first phase of flow analysis, in which flow
analysis "looks ahead" to see which variables are potentially assigned
in each code block, was failing to properly account for variables that
are assigned by pattern assignment expressions. This "look ahead"
information is used by flow analysis to determine the effects of
nonlinear control flow (e.g. to figure out which variables to demote
at the top of a loop, or to figure out which variables are potentially
assigned inside a closure). As a result, it was possible to construct
correct code that would be rejected by the analyzer and compiler, as
well as incorrect code that would (unsoundly) be accepted.

The fix is to call `AssignedVariables.write` from the analyzer's
`FlowAnalysisVisitor`, and from the CFE's `BodyBuilder`, to let flow
analysis know about variable assignments that occur inside pattern
assignment expressions.

Fixes: https://github.com/dart-lang/sdk/issues/52767.

Bug: https://github.com/dart-lang/sdk/issues/52745
Change-Id: If470f44a5e6b3afb03c1976a9609d884e2d3009c
Cherry-pick: https://dart-review.googlesource.com/c/sdk/+/310502
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/310702
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>