Buffer the entire http header to one packet, and buffer other data in chunks of 4-16 kb.

tests/standalone/io/http_content_length_test.dart

Index: tests/standalone/io/http_content_length_test.dart
diff --git a/tests/standalone/io/http_content_length_test.dart b/tests/standalone/io/http_content_length_test.dart
index 92aa3284f7bb46fee5fbbaad61ba8be9274cd2ba..cbe04eaa351e6af286a09459be91cae44fe9805c 100644
--- a/tests/standalone/io/http_content_length_test.dart
+++ b/tests/standalone/io/http_content_length_test.dart
@@ -130,6 +130,9 @@ void testBody(int totalConnections, bool useHeader) {
                   if (++clientCount == totalConnections) {
                     client.close();
                   }
+                },
+                onError: (error) {
+                  // Undefined what server response sends.
                 });
           });
     }