Gets a data-flow node where this value enters the current codebase.
For example:
# API::getTopLevelMember("Foo").asSource()
Foo
# API::getTopLevelMember("Foo").getMethod("bar").getReturn().asSource()
Foo.bar
# 'x' is found by:
# API::getTopLevelMember("Foo").getMethod("bar").getBlock().getParameter(0).asSource()
Foo.bar do |x|
end