fix #26512, correct handling of fuzzy arrows in strong mode 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 2594 matching lines @@
     Iterable<double> id;
     int x =
         /*info:ASSIGNMENT_CAST should be error:INVALID_ASSIGNMENT*/
         b ? li : id;
     return /*warning:DOWN_CAST_COMPOSITE should be pass*/b ? li : id;
   }
 }
 ''');
   }
 
+  void test_leastUpperBounds_fuzzyArrows() {
+    checkFile(r'''
+typedef String TakesA<T>(T item);
+
+void main() {
+  TakesA<int> f;
+  TakesA<dynamic> g;
+  TakesA<String> h;
+  g = h;
+  f = /*warning:DOWN_CAST_COMPOSITE*/f ?? g;
+}
+''');
+  }
+
   void test_loadLibrary() {
     addFile('''library lib1;''', name: '/lib1.dart');
     checkFile(r'''
 import 'lib1.dart' deferred as lib1;
 import 'dart:async' show Future;
 main() {
   Future f = lib1.loadLibrary();
 }''');
   }
 
@@ skipping 690 matching lines @@
   // There's no telling if that will work. For example, consider:
   //
   //     new SplayTreeMap<Uri>();
   //
   // This would end up calling .compareTo() on a Uri, which doesn't
   // define that since it doesn't implement Comparable.
   SplayTreeMap([int compare(K key1, K key2),
                 bool isValidKey(potentialKey)])
     : _comparator = /*warning:DOWN_CAST_COMPOSITE*/(compare == null) ? Comparable.compare : compare,
       _validKey = (isValidKey != null) ? isValidKey : ((v) => true) {
-    _Predicate<Object> v = (isValidKey != null)
+
+    // NOTE: this is a down cast because isValidKey has fuzzy arrow type.
+    _Predicate<Object> v = /*warning:DOWN_CAST_COMPOSITE*/(isValidKey != null)
         ? isValidKey : (/*info:INFERRED_TYPE_CLOSURE*/(_) => true);
 
     v = (isValidKey != null)
          ? v : (/*info:INFERRED_TYPE_CLOSURE*/(_) => true);
   }
 }
 void main() {
   Object obj = 42;
   dynamic dyn = 42;
   int i = 42;
@@ skipping 273 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();
 }
 ''');
   }
 }