[flow analysis] Simplify reachability computation in attachFinally.

Previously, `attachFinally` took advantage of
`Reachability.rebaseForward` to compute reachability after a
`try/finally` statement. This was overkill, since
`Reachability.rebaseForward` contains special logic to account for
widely diverging reachability stacks, whereas `attachFinally` always
deals with reachability stacks that have an immediate common parent.

This change adds an assertion to verify that the reachability stacks
always have an immediate common ancestor, and replaces the call to
`Reachability.rebaseForward` with the equivalent direct logic.

Making this change will simplify the process of writing a spec for
flow analysis.

Change-Id: Icd7d3ebc150d1836932efe3891b3ea1077134351
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/437300
Reviewed-by: Chloe Stefantsova <[email protected]>
Commit-Queue: Paul Berry <[email protected]>

Author: stereotype441

pkg/_fe_analyzer_shared/lib/src/flow_analysis/flow_analysis.dart

@@ -2708,7 +2708,11 @@ class FlowModel<Type extends Object> {
 
     // Code that follows the `try/finally` is reachable iff the end of the `try`
     // block is reachable _and_ the end of the `finally` block is reachable.
-    Reachability newReachable = afterFinally.reachable.rebaseForward(reachable);
+    assert(identical(reachable.parent, afterFinally.reachable.parent));
+    Reachability newReachable =
+        afterFinally.reachable.locallyReachable
+            ? reachable
+            : reachable.setUnreachable();
 
     // Consider each promotion key that is common to all three models.
     FlowModel<Type> result = setReachability(newReachable);