[cfe] Fix type field promotion on LHS of ??.

The CFE was passing the original LHS expression to
`flowAnalysis.ifNullExpression_rightBegin`. This was incorrect; it
needs to pass the rewritten LHS expression. Most of the time this was
harmless, since in most cases:

- The expression on the LHS of `??` was not promotable, in which case
  passing the wrong expression didn't matter,

- Or the expression on the of `??` was a read of a local variable, in
  which case there was no problem, since local variable reads don't
  require rewriting.

However, in cases where the LHS of `??` is a read of a promotable
property, it does matter, since property reads are often rewritten
during inference.

Fixes #56874.

Bug: #56874
Change-Id: Ie42b96b0bc14faec38e8dbcb9e11534d4a1db0dc
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/389140
Commit-Queue: Paul Berry <[email protected]>
Reviewed-by: Johnni Winther <[email protected]>
Reviewed-by: Chloe Stefantsova <[email protected]>

Author: stereotype441

pkg/front_end/lib/src/type_inference/inference_visitor.dart

@@ -2029,7 +2029,7 @@ class InferenceVisitorImpl extends InferenceVisitorBase
     // This ends any shorting in `node.left`.
     Expression left = lhsResult.expression;
 
-    flowAnalysis.ifNullExpression_rightBegin(node.left, new SharedTypeView(t1));
+    flowAnalysis.ifNullExpression_rightBegin(left, new SharedTypeView(t1));
 
     // - Let `T2` be the type of `e2` inferred with context type `J`, where:
     //   - If `K` is `_` or `dynamic`, `J = T1`.

tests/language/inference_update_2/field_promotion_on_lhs_of_if_null_test.dart

@@ -0,0 +1,26 @@
+// Copyright (c) 2024, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// Tests that a promotable field access appearing on the LHS of `??` is properly
+// promoted in a code path where it is known to be non-null.
+
+import 'package:expect/expect.dart';
+import '../static_type_helper.dart';
+
+class A {
+  final int? _f1;
+
+  A(this._f1);
+}
+
+test(A a) {
+  a._f1.expectStaticType<Exactly<int?>>();
+  a._f1 ?? [a._f1.expectStaticType<Exactly<int?>>(), throw ''];
+  a._f1.expectStaticType<Exactly<int>>();
+}
+
+main() {
+  test(A(0));
+  Expect.throws(() => test(A(null)));
+}