Possible bug in the construct/deconstruct_and_eval internals

Contructs of form `dt[, a := b$c()]` fail (data.table version 1.9.3). A more detailed example taken from http://stackoverflow.com/questions/25383110 would be

```
RF <- setRefClass(
  Class = "RF",
  methods = list(
    get = function() { "/foo/bar" }
  )
)
rf <- RF$new()

mdt <- data.table( x= c("a", "b"), y = 1:2 )
mdt[ , z := rf$get() ] # gives warning

mdt$z
```
