Migrate block 163 to Dart 2.0.

tests/language_2/unresolved_top_level_var_test.dart

 // Copyright (c) 2011, 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 an unresolved identifier at the top level does not crash
-// the parser.
+// Test that an unresolved identifier at the top level causes a compile-time
+// error.
 
-var a = b;
+var a = b; /*@compile-error=unspecified*/
 
 main() {
   print(a);
-}
+}