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

sdk/lib/io/http_headers.dart

Index: sdk/lib/io/http_headers.dart
diff --git a/sdk/lib/io/http_headers.dart b/sdk/lib/io/http_headers.dart
index b32748beda41dd683eea8cbd26dc28d6061cf705..62e940c2ffcb18ca9932baf885f6bcc291db21e8 100644
--- a/sdk/lib/io/http_headers.dart
+++ b/sdk/lib/io/http_headers.dart
@@ -123,12 +123,16 @@ class _HttpHeaders implements HttpHeaders {
           "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;
+      }
     }
   }