Set the system identifier for this input source.
The system identifier is optional if there is a byte stream or a character stream, but it is still useful to provide one, since the application can use it to resolve relative URIs and can include it in error messages and warnings (the parser will attempt to open a connection to the URI only if there is no byte stream or character stream specified).
If the application knows the character encoding of the object pointed to by the system identifier, it can register the encoding using the setEncoding method.
If the system identifier is a URL, it must be fully resolved (it may not be a relative URL).
|The system identifier as a string.|