Eliminate old style import from language tests

tests/language/import_core_prefix_test.dart

Index: tests/language/import_core_prefix_test.dart
===================================================================
--- tests/language/import_core_prefix_test.dart	(revision 15598)
+++ tests/language/import_core_prefix_test.dart	(working copy)
@@ -3,8 +3,8 @@
 // BSD-style license that can be found in the LICENSE file.
 // Test explicit import of dart:core in the source code..
 
-#library("ImportCorePrefixTest.dart");
-#import("dart:core", prefix:"mycore");
+library ImportCorePrefixTest.dart;
+import "dart:core" as mycore;
 
 class Object {
 }