Report compile-time error on disallowed metadata values.

tests/compiler/dart2js/mirrors_helper.dart

 // Copyright (c) 2012, 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.
 
 /**
  * This file is read by 'mirrors_test.dart'.
  */
 
 library mirrors_helper;
 
 typedef E Func<E,F extends Foo>(F f);
 
 main() {
 
 }
 
 /// Singleline doc comment.
-@Metadata // This is intentionally the type literal.
+@Metadata(null)
 // Singleline comment 1.
 // Singleline comment 2.
-@Metadata(null)
 @Metadata(true)
 @Metadata(false)
 @Metadata(0)
 @Metadata(1.5)
 @Metadata("Foo")
 @Metadata(const ["Foo"])
 @Metadata(const {'foo':"Foo"})
 @metadata
 /** Multiline doc comment. */
 /* Multiline comment. */ class Foo {
@@ skipping 38 matching lines @@
   _PrivateClass._privateConstructor();
   factory _PrivateClass._privateFactoryConstructor() => null;
 }
 
 const metadata = const Metadata(null);
 
 class Metadata {
   final data;
   const Metadata(this.data);
 }