[flow analysis] Make split/conseravativeJoin order more uniform.

While doing specification work for flow analysis, I noticed that in
circumstances where a flow model needs to have both the `split` and
`conservativeJoin` operations applied to it, sometimes flow analysis
applied `split` first, and sometimes it applied `conservativeJoin`
first.

The actual order doesn't matter, since `split` only affects the flow
model's reachability, and `conservativeJoin` only affects the flow
model's variable info. But for the purpose of making the specification
clearer, it's nice to be consistent. This change adjusts the order so
that `split` always happens before `conservativeJoin`.

Change-Id: Ia5dfc9335ee85f122ae274f6bb850f83f12b88d2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/437060
Reviewed-by: Konstantin Shcheglov <[email protected]>
Commit-Queue: Paul Berry <[email protected]>

Author: stereotype441

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

@@ -5525,8 +5525,11 @@ class _FlowAnalysisImpl<
       _current.reachable,
     );
     _stack.add(context);
-    _current =
-        _current.conservativeJoin(this, info.written, info.captured).split();
+    _current = _current.split().conservativeJoin(
+      this,
+      info.written,
+      info.captured,
+    );
     _statementToContext[doStatement] = context;
   }
 
@@ -5667,8 +5670,11 @@ class _FlowAnalysisImpl<
   @override
   void for_conditionBegin(Node node) {
     AssignedVariablesNodeInfo info = _assignedVariables.getInfoForNode(node);
-    _current =
-        _current.conservativeJoin(this, info.written, info.captured).split();
+    _current = _current.split().conservativeJoin(
+      this,
+      info.written,
+      info.captured,
+    );
   }
 
   @override
@@ -5694,8 +5700,11 @@ class _FlowAnalysisImpl<
   @override
   void forEach_bodyBegin(Node node) {
     AssignedVariablesNodeInfo info = _assignedVariables.getInfoForNode(node);
-    _current =
-        _current.conservativeJoin(this, info.written, info.captured).split();
+    _current = _current.split().conservativeJoin(
+      this,
+      info.written,
+      info.captured,
+    );
     _SimpleStatementContext<Type> context = new _SimpleStatementContext<Type>(
       _current.reachable.parent!,
       _current,