Fix crash with generalized function types in type tests.

pkg/front_end/testcases/function_type_is_check.dart

+// Copyright (c) 2017, 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 "package:expect/expect.dart" show Expect;
+
+test(f) {
+  if (f is void Function(Object, StackTrace)) return 1;
+  if (f is void Function(Object)) return 10;
+  if (f is void Function()) return 100;
+}
+
+main() {
+  Expect.equals(
+      111,
+      test(() => null) +
+          test((Object o) => null) +
+          test((Object o, StackTrace t) => null));
+}