test rename overhaul: step 8 - language tests

tests/language/prefix_test1.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("PrefixTest1.dart");
 
-#import("PrefixTest2.dart", prefix: "prefix");
+#import("prefix_test2.dart", prefix: "prefix");
 class Prefix {
   static final int foo = 43;
 
   static getSource() {
     return foo;
   }
 
   static getImport() {
     return prefix.Prefix.foo;
   }
 }