Overloaded += and -= for MutableCollection:
Actually this may change the behavior of code in some cases, e.g.
var x = arrayListOf("a")
x += "b" // was the same as x = x + "b"
(created NEW list and concatenated, stored in var, not changed the object)

After my changes x += "b" actually changes the object,
as IMO it's expected to do so.
Assignment operations: added plusAssing for dom.Node