fix #27408, reverts "ignore dynamic in inference"

pkg/dev_compiler/test/codegen/language/regress_21998_4_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.
 
 import 'dart:math' as Math;
 import 'package:expect/expect.dart';
 import 'regress_21998_lib1.dart' as lib1;
 import 'regress_21998_lib2.dart';
 import 'regress_21998_lib3.dart';
 
 main() {
   Expect.equals(4, new C().m());
 }
 
 class C {
   max(a) => a;
 
   m() {
-    return max(Math.max(
+    return max(Math.max/*<num>*/(
                lib3_max(0, lib2_max(1, 2)), lib1.max('a', 'b', 'cd').length));
   }
 }