Switch from interfaces to abstract classes in dart:io.

tests/standalone/io/http_headers_test.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.
 
 #import('dart:math');
 
 #source("../../../runtime/bin/input_stream.dart");
 #source("../../../runtime/bin/output_stream.dart");
 #source("../../../runtime/bin/chunked_stream.dart");
 #source("../../../runtime/bin/string_stream.dart");
@@ skipping 220 matching lines @@
     if (parameters != null) {
       Expect.equals(parameters.length, contentType.parameters.length);
       parameters.forEach((String name, String value) {
         Expect.equals(value, contentType.parameters[name]);
       });
     } else {
       Expect.equals(0, contentType.parameters.length);
     }
   }
 
-  _ContentType contentType;
-  contentType = new _ContentType();
+  ContentType contentType;
+  contentType = new ContentType();
   Expect.equals("", contentType.primaryType);
   Expect.equals("", contentType.subType);
   Expect.equals("/", contentType.value);
   contentType.value = "text/html";
   Expect.equals("text", contentType.primaryType);
   Expect.equals("html", contentType.subType);
   Expect.equals("text/html", contentType.value);
 
   contentType = new _ContentType.fromString("text/html");
   check(contentType, "text", "html");
@@ skipping 149 matching lines @@
   testIfModifiedSince();
   testHost();
   testEnumeration();
   testHeaderValue();
   testContentType();
   testContentTypeCache();
   testCookie();
   testInvalidCookie();
   testHeaderLists();
 }