Located in File: /vendors/swiftMailer/classes/Swift/ByteStream/ArrayByteStream.php
If $stack is given the stream will be populated with the bytes it contains.
The stream acts as an observer, receiving all data that is written. All write() and flushBuffers() operations will be mirrored.
If $is is not bound, no errors will be raised. If the stream currently has any buffered data it will be written to $is before unbinding occurs.