Introduce (private) ZLibFilter to dart:io.

runtime/bin/filter_impl.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.
+
+
+class _FilterImpl implements Filter {
+  _FilterImpl() {
+  }
+
+  Future<List<int>> process(List<int> data) {
+    var c = new Completer<List<int>>();
+    try {
+      c.complete(_Process(data));
+    } catch (e) {
+      c.completeException(e);
+    }
+    return c.future;
+  }
+
+  void end() {
+    _End();
+  }
+
+  List<int> _Process(List<int> data) native "Filter_Process";
+  void _End() native "Filter_End";
+}
+
+class _ZLibFilterImpl extends _FilterImpl {
+  _ZLibFilterImpl() {
+    var result = _Init();
+    if (result is Exception) {
+      throw result;  // ???
+    }
+  }
+
+  bool _Init() native "Filter_CreateZLIB";
+}