Eliminate old style import from language tests

tests/language/prefix18_negative_test.dart

Index: tests/language/prefix18_negative_test.dart
===================================================================
--- tests/language/prefix18_negative_test.dart	(revision 15598)
+++ tests/language/prefix18_negative_test.dart	(working copy)
@@ -2,10 +2,10 @@
 // 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.
 //
-// prefix: strings should only contain valid identifiers
+// prefix must be a valid identifier
 
-#library("Prefix18NegativeTest.dart");
-#import("library1.dart", prefix:"lib1.invalid");
+library Prefix18NegativeTest.dart;
+import "library1.dart" as lib1.invalid;
 
 main() {
 }