lawn_isolines is broken

[sckott]

with new turf 4.5.2 doesn't work

library(lawn)
pts <- lawn_point_grid(c(0, 30, 20, 50), 100, 'miles')
pts$features$properties <-
   data.frame(temperature = round(rnorm(NROW(pts$features), mean = 5)),
   stringsAsFactors = FALSE)
breaks <- c(0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
lawn_isolines(points = pts, breaks, z = 'temperature')
#> Error in context_eval(join(src), private$context) : 
#>   TypeError: Object function Object() { [native code] } has no method 'assign'

tests commented out for now.

[sckott]

still broken on turf 4.7.3