[flow analysis] Add offset parameters to API.

Adds optional `offset` parameters to most flow analysis methods. Later
CLs will use these offsets to create a log of flow analysis state
changes, which can be later queried in order to determine the flow
analysis state at a given source code location.

A few new methods are added:

- `checkOffset`, which can be used by the client to help verify that
  the offsets it passes to flow analysis are sensible.

- `patternAssignment_beforeRhs`, which the client should call to
  inform flow analysis that it's about to visit the RHS of a pattern
  assignment expression.

- `patternForIn_beforeExpression`, which the client should call to
  inform flow analysis that it's about to visit the iterable
  expression in a pattern "for-in" statement or collection element.

- `patternVariableDeclaration_beforeInitializer`, which the client
  should call to inform flow analysis that it's about to visit the
  initializer of a pattern variable declaration.

The three new `pattern...` methods don't currently do anything, but
they will be needed once we try to populate the log. They will be
needed because in these three constructs, type analysis visits the
expression on the RHS _before_ it visits the pattern on the LHS. So if
the pattern match causes any promotions to occur, the log will need to
reflect the fact that those promotions are not in effect during
evaluation of the expression.

Part of https://github.com/dart-lang/sdk/issues/63816.

Change-Id: Ia920d708665e2bd85e84fc8cd7f826986a6a6964
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/529080
Reviewed-by: Chloe Stefantsova <cstefantsova@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>