Update metadata syntax test to check for non-existing constants and type literals (see issue 14647).

tests/language/metadata_syntax_test.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.
 
 import "package:expect/expect.dart";
 
 // Test that is is an error to use type arguments in metadata without
 // parens.
 
 @Bar()
 @List<String> /// 01: compile-time error
 class Bar {
   @Bar()
   @List<String> /// 02: compile-time error
   const Bar();
 
   @Bar()
   @List<String> /// 03: compile-time error
-  final x = '';
+  final x = 'x';
+
+  @unknown /// 04: compile-time error
+  final y = 'y';
+
+  @Bar /// 05: compile-time error
+  final z = 'z';
+
+  @1234 /// 06: compile-time error
+  final w = 'w';
 }
 
 main() {
-  Expect.equals('', new Bar().x);
-  Expect.equals('', const Bar().x);
+  Expect.equals('x', new Bar().x);
+  Expect.equals('x', const Bar().x);
+  Expect.equals('y', new Bar().y);
+  Expect.equals('y', const Bar().y);
+  Expect.equals('z', new Bar().z);
+  Expect.equals('z', const Bar().z);
+  Expect.equals('z', new Bar().z);
+  Expect.equals('z', const Bar().z);
+  Expect.equals('w', new Bar().w);
+  Expect.equals('w', const Bar().w);
 }