Eliminate old style import from 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");
+library PrefixTest1.dart;
 
-#import("prefix_test2.dart", prefix: "prefix");
+import "prefix_test2.dart" as prefix;
 class Prefix {
   static const int foo = 43;
 
   static getSource() {
     return foo;
   }
 
   static getImport() {
     return prefix.Prefix.foo;
   }
 }