A ClosableStringSink extends the StringSink interface by adding a
close method.
sink and a callback
onClose which is invoked when the returned instance is closed.
this and flushes any outstanding data.
obj to a String by invoking Object.toString and
adds the result to this.
objects and writes them in sequence.
charCode to this. [...]
obj to a String by invoking Object.toString and
adds the result to this, followed by a newline.