Slightly speed up http-parser and http-header-writing.

sdk/lib/io/http_impl.dart

 // Copyright (c) 2013, 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.
 
 part of dart.io;
 
 const int _HEADERS_BUFFER_SIZE = 8 * 1024;
 
 class _HttpIncoming extends Stream<List<int>> {
   final int _transferLength;
@@ skipping 1315 matching lines @@
       request.headers.set(HttpHeaders.AUTHORIZATION, "Basic $auth");
     } else {
       // Look for credentials.
       creds = _httpClient._findCredentials(uri);
       if (creds != null) {
         creds.authorize(request);
       }
     }
     // Start sending the request (lazy, delayed until the user provides
     // data).
-    _httpParser.responseToMethod = method;
+    _httpParser.isHead = method == "HEAD";
     _streamFuture = outgoing.done
         .then((s) {
           // Request sent, set up response completer.
           _nextResponseCompleter = new Completer();
 
           // Listen for response.
           _nextResponseCompleter.future
               .then((incoming) {
                 _currentUri = null;
                 incoming.dataDone.then((closing) {
@@ skipping 1221 matching lines @@
   const _RedirectInfo(this.statusCode, this.method, this.location);
 }
 
 String _getHttpVersion() {
   var version = Platform.version;
   // Only include major and minor version numbers.
   int index = version.indexOf('.', version.indexOf('.') + 1);
   version = version.substring(0, index);
   return 'Dart/$version (dart:io)';
 }