test rename overhaul: step 8 - language tests

tests/language/prefix21_good_lib.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.
 
 #library("Prefix21Good");
 
 int goodFunction(int x) {
   return x;
 }
 
 Function get getValue() {
   return goodFunction;
 }