Fix typechecks for constant assignments of function to typedefs.

tests/language/type_check_const_function_typedef_test.dart

Index: tests/language/type_check_const_function_typedef_test.dart
diff --git a/tests/compiler/dart2js_extra/16407_test.dart b/tests/language/type_check_const_function_typedef_test.dart
similarity index 51%
copy from tests/compiler/dart2js_extra/16407_test.dart
copy to tests/language/type_check_const_function_typedef_test.dart
index aff1ba337ddab1eb5d219ac635653b48d4f8d1bc..484c61aeef1a2f3230d5e97c1e6af609b5bac1b0 100644
--- a/tests/compiler/dart2js_extra/16407_test.dart
+++ b/tests/language/type_check_const_function_typedef_test.dart
@@ -2,15 +2,21 @@
 // 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.
 
+// Tests that typechecks on const objects with typedefs work.
+
 import "package:expect/expect.dart";
 
-// Regression test for Issue 16407.
+typedef String Int2String(int x);
 
-void main() {
-  foo(null, true);
-  foo('x', false);
+class A {
+  final Int2String f;
+  const A(this.f);
 }
 
-var foo = (x, result) {
-  Expect.equals(result, x is Null, '$x is Null');
-};
+String foo(int x) => "str";
+
+const a = const A(foo);
+
+main() {
+  Expect.equals("str", a.f(499));
+}