Prepending node without `contentEditable` to node with `contenteditable True` throws runtime exception.

[Damirados]

Firefox 52 and chromium 57, elm 0.18, linux

Here is example code, Prepend non editable will throw exception:

import Html exposing (beginnerProgram, div, button, text, span)
import Html.Attributes exposing (contenteditable)
import Html.Events exposing (onClick)

main =
  beginnerProgram { model = [Editable], view = view, update = (::) }

type Element = NonEditable | Editable

view model =
  div []
    [ button [ onClick NonEditable ] [ text "Prepend non editable" ]
    , button [ onClick Editable ] [ text "Prepend editable" ]
    , div [] (List.map viewElement model)
    ]

viewElement element =
  case element of
    NonEditable ->
      span [] [ text "non editable" ]

    Editable ->
      span [ contenteditable True ] [ text "editable" ]

works with this code

viewElement element =
  case element of
    NonEditable ->
      span [ contenteditable False ] [ text "non editable" ]

    Editable ->
      span [ contenteditable True ] [ text "editable" ]

[process-bot]

Thanks for the issue! Make sure it satisfies this checklist. My human colleagues will appreciate it!

Here is what to expect next, and if anyone wants to comment, keep these things in mind.

[evancz]

I am consolidating all contenteditable stuff into #104, so we can see it all in one place.

Thanks for the report!