Introduce (private) ZLibFilter to dart:io.

runtime/bin/filter.dart

 // Copyright (c) 2012, 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('source_mapping_crash_test');
 
-#source('source_mapping_crash_source.dart');
+abstract class Filter {
+  Future<List<int>> process(List<int> data);
 
-class Sub extends Super {
-  Sub(var x) : super(x.y);
+  // FIXME: Rename to 'done'?
+  void end();
 }
 
-class X { var y; }
+class ZLibFilter implements Filter {
+  factory ZLibFilter() => new _ZLibFilterImpl();
+}
 
-main() {
-  new Sub(new X());
-}