[cfe] Make type inference for if-null expressions consistent with analyzer.

Fixes https://github.com/dart-lang/language/issues/3650.
Fixes https://github.com/dart-lang/sdk/issues/55436.

Bug: https://github.com/dart-lang/language/issues/3650
Change-Id: I30b39221c85713aab10f2edc35625f38e34cae5e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/362100
Reviewed-by: Chloe Stefantsova <cstefantsova@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>

CHANGELOG.md

@@ -7,7 +7,15 @@
   match the behavior of the analyzer. This change is not expected to make any
   difference in practice.
 
+- **Breaking Change** [#55436][]: The context used by the compiler to perform
+  type inference on the right hand side of an "if-null" expression (`e1 ?? e2`)
+  has been changed to match the behavior of the analyzer. change is expected to
+  have low impact on real-world code. But in principle it could cause
+  compile-time errors or changes in runtime behavior by changing inferred
+  types. The old behavior can be restored by supplying explicit types.
+
 [#55418]: https://github.com/dart-lang/sdk/issues/55418
+[#55436]: https://github.com/dart-lang/sdk/issues/55436
 
 ### Dart Runtime
 

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

@@ -1931,9 +1931,9 @@ class InferenceVisitorImpl extends InferenceVisitorBase
     flowAnalysis.ifNullExpression_rightBegin(node.left, t1);
 
     // - Let `T2` be the type of `e2` inferred with context type `J`, where:
-    //   - If `K` is `_`, `J = T1`.
+    //   - If `K` is `_` or `dynamic`, `J = T1`.
     DartType j;
-    if (typeContext is UnknownType) {
+    if (typeContext is UnknownType || typeContext is DynamicType) {
       j = t1;
     } else
     //   - Otherwise, `J = K`.

tests/language/if_null/if_null_with_dynamic_context_test.dart

@@ -0,0 +1,14 @@
+// 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.
+
+// Verify that if an if-null expression is analyzed in a `dynamic` context, the
+// context for the RHS is taken from the static type of the LHS (see
+// https://github.com/dart-lang/language/issues/3650).
+
+import '../static_type_helper.dart';
+
+main() async {
+  dynamic x =
+      (null as List<int>?) ?? ([]..expectStaticType<Exactly<List<int>>>());
+}