Switch from interfaces to abstract classes in dart:io.

runtime/bin/http_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 _HttpHeaders implements HttpHeaders {
   _HttpHeaders() : _headers = new Map<String, List<String>>();
 
   List<String> operator[](String name) {
     name = name.toLowerCase();
     return _headers[name];
@@ skipping 409 matching lines @@
     expect(";");
     parseParameters();
   }
 
   String value;
   Map<String, String> _parameters;
 }
 
 
 class _ContentType extends _HeaderValue implements ContentType {
-  _ContentType([String primaryType = "", String subType = ""])
-    : _primaryType = primaryType, _subType = subType;
+  _ContentType(String primaryType, String subType)
+      : _primaryType = primaryType, _subType = subType, super("");
 
   _ContentType.fromString(String value) : super.fromString(value);
 
   String get value => "$_primaryType/$_subType";
 
   void set value(String s) {
     int index = s.indexOf("/");
     if (index == -1 || index == (s.length - 1)) {
       primaryType = s.trim().toLowerCase();
       subType = "";
@@ skipping 1737 matching lines @@
 
 
 class _RedirectInfo implements RedirectInfo {
   const _RedirectInfo(int this.statusCode,
                       String this.method,
                       Uri this.location);
   final int statusCode;
   final String method;
   final Uri location;
 }