Eliminate old style import from language tests

tests/language/prefix21_test.dart

Index: tests/language/prefix21_test.dart
===================================================================
--- tests/language/prefix21_test.dart	(revision 15598)
+++ tests/language/prefix21_test.dart	(working copy)
@@ -2,9 +2,9 @@
 // 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("Prefix21");
-#import("prefix21_good_lib.dart", prefix: "good");
-#import("prefix21_bad_lib.dart", prefix: "bad");
+library Prefix21;
+import "prefix21_good_lib.dart" as good;
+import "prefix21_bad_lib.dart" as bad;
 
 main() {
   Expect.equals(good.getValue(42), 42);