[dart2js] Update MethodTypeInformation to narrow the return type of

_lateReadCheck.

Change-Id: If76a5b34cfd9da9721bfa0d8642b018a4af131dd
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/213901
Reviewed-by: Stephen Adams <sra@google.com>
Commit-Queue: Mayank Patke <fishythefish@google.com>

pkg/compiler/lib/src/inferrer/type_graph_nodes.dart

@@ -625,6 +625,9 @@ class MethodTypeInformation extends MemberTypeInformation {
   @override
   AbstractValue _potentiallyNarrowType(
       AbstractValue mask, InferrerEngine inferrer) {
+    if (inferrer.commonElements.isLateReadCheck(_method)) {
+      mask = inferrer.abstractValueDomain.excludeLateSentinel(mask);
+    }
     return _narrowType(inferrer.closedWorld, mask, _type.returnType);
   }
 

pkg/compiler/test/codegen/late_test.dart

@@ -0,0 +1,36 @@
+// Copyright (c) 2021, 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.
+
+import 'dart:async';
+import 'package:async_helper/async_helper.dart';
+import '../helpers/compiler_helper.dart';
+
+const String TEST = r"""
+class Foo {
+  late int x;
+}
+
+int foo() {
+  final foo = Foo();
+  foo.x = 40;
+  return foo.x + 2;
+  // present: '+ 2'
+  // absent: 'add'
+}
+""";
+
+Future check(String test) {
+  return compile(test,
+      entry: 'foo',
+      check: checkerForAbsentPresent(test),
+      disableTypeInference: false,
+      disableInlining: false,
+      soundNullSafety: true);
+}
+
+void main() {
+  asyncTest(() async {
+    await check(TEST);
+  });
+}