Gets a data-flow node where this value potentially flows into an external library.
This is usually the argument of a call, but can also be the return value of a callback.
For example:
# 'x' is found by:
# API::getTopLevelMember("Foo").getMethod("bar").getParameter(0).asSink()
Foo.bar(x)
Foo.bar(-> {
# 'x' is found by:
# API::getTopLevelMember("Foo").getMethod("bar").getParameter(0).getReturn().asSink()
x
})