Fix native extension lookup

tests/standalone/io/test_relative_extension.dart

-// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// Copyright (c) 2016, 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 test_extension;
 
-import "dart-ext:test_extension";
+import "dart-ext:extension/test_extension";
 
 class Cat {
   Cat(this.x);
 
   num x;
 
   String toString() => 'cat $x';
 
   // Implements (a != null) ? a : b using a native C++ function and the API.
   static int ifNull(a, b) native 'TestExtension_IfNull';
 
   static int throwMeTheBall(ball) native 'TestExtension_ThrowMeTheBall';
 }