test rename overhaul: step 8 - language tests

tests/language/prefix_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.
 
 #library("PrefixTest.dart");
-#import("PrefixTest1.dart");
+#import("prefix_test1.dart");
 
 class PrefixTest {
   static testMain() {
     Expect.equals(Prefix.getSource(), Prefix.getImport() + 1);
   }
 }
 
 main() {
   PrefixTest.testMain();
 }