Ensure that try/catch statements promote properly in unreachable code.

Bug: https://github.com/dart-lang/sdk/issues/40009
Change-Id: Ie4b8809c6f37ff67891bc874c0c0ec10c70eb9fa
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/166660
Reviewed-by: Johnni Winther <johnniwinther@google.com>

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

@@ -3255,6 +3255,7 @@ class _FlowAnalysisImpl<Node, Statement extends Node, Expression, Variable,
 
   @override
   void tryCatchStatement_bodyBegin() {
+    _current = _current.split();
     _stack.add(new _TryContext<Variable, Type>(_current));
   }
 
@@ -3301,7 +3302,7 @@ class _FlowAnalysisImpl<Node, Statement extends Node, Expression, Variable,
   void tryCatchStatement_end() {
     _TryContext<Variable, Type> context =
         _stack.removeLast() as _TryContext<Variable, Type>;
-    _current = context._afterBodyAndCatches;
+    _current = context._afterBodyAndCatches.unsplit();
   }
 
   @override

pkg/_fe_analyzer_shared/test/flow_analysis/type_promotion/data/promotion_in_dead_code.dart

@@ -201,3 +201,23 @@ switchPromoteInImplicitDefault(Object o, int i, Object p) {
   }
   /*int*/ o;
 }
+
+tryCatchPromoteInTry(Object o) {
+  return;
+  try {
+    if (o is! int) return;
+  } catch (_) {
+    return;
+  }
+  /*int*/ o;
+}
+
+tryCatchPromoteInCatch(Object o) {
+  return;
+  try {
+    return;
+  } catch (_) {
+    if (o is! int) return;
+  }
+  /*int*/ o;
+}