Get information about invisible built-in functions

How can we see the code of invisible functions, i.e. mosaicplot? 

> mosaicplot

yields only:

function (x, ...)
UseMethod("mosaicplot")


but the functions "methods" and "getAnywhere" do the trick:

> methods(mosaicplot)

[1] mosaicplot.default* mosaicplot.formula*

Non-visible functions are asterisked

> getAnywhere("mosaicplot.formula")
A single object matching ‘mosaicplot.formula’ was found
It was found in the following places
registered S3 method for mosaicplot from namespace graphics
namespace:graphics
with value

function (formula, data = NULL, ..., main = deparse(substitute(data)),
subset, na.action = stats::na.omit)
{
main
m <- match.call(expand.dots = FALSE)
.....