Fix http headers to enable chunked transfer encoding when content-length is set to -1.

sdk/lib/io/http_headers.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;
 
 class _HttpHeaders implements HttpHeaders {
   final Map<String, List<String>> _headers;
   final String protocolVersion;
 
@@ skipping 105 matching lines @@
 
   void set contentLength(int contentLength) {
     _checkMutable();
     if (protocolVersion == "1.0" &&
         persistentConnection &&
         contentLength == -1) {
       throw new HttpException(
           "Trying to clear ContentLength on HTTP 1.0 headers with "
           "'Connection: Keep-Alive' set");
     }
+    if (_contentLength == contentLength) return;
     _contentLength = contentLength;
     if (_contentLength >= 0) {
       if (chunkedTransferEncoding) chunkedTransferEncoding = false;
       _set(HttpHeaders.CONTENT_LENGTH, contentLength.toString());
     } else {
       removeAll(HttpHeaders.CONTENT_LENGTH);
+      if (protocolVersion == "1.1") {
+        chunkedTransferEncoding = true;
+      }
     }
   }
 
   bool get chunkedTransferEncoding => _chunkedTransferEncoding;
 
   void set chunkedTransferEncoding(bool chunkedTransferEncoding) {
     _checkMutable();
     if (chunkedTransferEncoding && protocolVersion == "1.0") {
       throw new HttpException(
           "Trying to set 'Transfer-Encoding: Chunked' on HTTP 1.0 headers");
@@ skipping 697 matching lines @@
   void clear() {
     throw new UnsupportedError("Cannot modify an unmodifiable map");
   }
   void forEach(void f(K key, V value)) => _map.forEach(f);
   Iterable<K> get keys => _map.keys;
   Iterable<V> get values => _map.values;
   int get length => _map.length;
   bool get isEmpty => _map.isEmpty;
   bool get isNotEmpty => _map.isNotEmpty;
 }