fix #27036, pass definite function types to LUB

pkg/analyzer/test/src/task/strong/checker_test.dart

 // Copyright (c) 2015, 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.
 
 library analyzer.test.src.task.strong.checker_test;
 
 import '../../../reflective_tests.dart';
 import 'strong_test_helper.dart';
 
 void main() {
@@ skipping 2959 matching lines @@
   b = b ?? new A();
 
   // downwards inference
   C<int> c, d;
   c ??= /*info:INFERRED_TYPE_ALLOCATION*/new C();
   d = d ?? /*info:INFERRED_TYPE_ALLOCATION*/new C();
 }
 ''');
   }
 
+  void test_nullCoalescingStrictArrow() {
+    checkFile(r'''
+bool _alwaysTrue(x) => true;
+typedef bool TakesA<T>(T t);
+class C<T> {
+  TakesA<T> g;
+  C(TakesA<T> f)
+    : g = f ?? _alwaysTrue;
+  C.a() : g = _alwaysTrue;
+}
+    ''');
+  }
+
   void test_privateOverride() {
     addFile(
         '''
 import 'main.dart' as main;
 
 class Base {
   var f1;
   var _f2;
   var _f3;
   get _f4 => null;
@@ skipping 670 matching lines @@
     // Regression test for https://github.com/dart-lang/sdk/issues/25069
     checkFile('''
 typedef int Foo();
 void foo() {}
 void main () {
   Foo x = /*error:INVALID_ASSIGNMENT,info:USE_OF_VOID_RESULT*/foo();
 }
 ''');
   }
 }