Eliminate old style import from language tests

tests/language/prefix_test1.dart

Index: tests/language/prefix_test1.dart
===================================================================
--- tests/language/prefix_test1.dart	(revision 15598)
+++ tests/language/prefix_test1.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("PrefixTest1.dart");
+library PrefixTest1.dart;
 
-#import("prefix_test2.dart", prefix: "prefix");
+import "prefix_test2.dart" as prefix;
 class Prefix {
   static const int foo = 43;