Fix bug with reflection on VM-internal native methods.

tests/language/vm/reflect_core_vm_test.dart

 // Copyright (c) 2013, 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.
 
 // Test reflection of private functions in core classes.
 
 import "package:expect/expect.dart";
 import "dart:mirrors";
 
 main() {
   var s = "string";
   var im = reflect(s);
-  try {
-    im.invoke(const Symbol("_setAt"), [0, 65]);
-    Expect.isTrue(false);  // Unreachable.
-  } catch (e) {
-    Expect.equals(true, e is NoSuchMethodError);
-  }
+  Expect.throws(() => im.invoke(const Symbol("_setAt"), [0, 65]), (e) => e is NoSuchMethodError);
 }