Reword an error message, and test that the error is produced.

dart/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.
+
+// 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 = '';
+}
+
+main() {
+  Expect.equals('', new Bar().x);
+  Expect.equals('', const Bar().x);
+}