Flow analysis: implement the new attachFinally algorithm.

When dart-lang/language#1274 (Infer
non-nullability from local boolean variables) is fixed, this algorithm
will replace the `restrict` algorithm is currently used at the bottom
of a try/finally statement to combine the flow models from the `try`
and `finally` blocks.  The new algorithm has very similar behavior to
the old one, however since it is based on SSA nodes it (a) is able to
make slightly more promotions than the old one, and (b) will be able
to properly update SSA nodes so that local boolean variables assigned
inside a `try` block will be able to used for promotion after the
`finally` block.

The new functionality is hidden behind a flag for now, so there's no
customer-visible change yet.

Bug: dart-lang/language#1274
Change-Id: I1d37f981688f58da1e5c6c7eee48f2319c997cef
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/174960
Reviewed-by: Johnni Winther <[email protected]>