message: make DocumentOffset compatible with what is returned from dom/getCaret
  so we can return a DocumentOffset from there without breakage
diff: @@ -15,12 +15,12 @@ limitations under the License.
  */
  export default class DocumentOffset {
  - constructor(offset, atEnd) {
  + constructor(offset, atNodeEnd) {
  this.offset = offset;
  - this.atEnd = atEnd;
  + this.atNodeEnd = atNodeEnd;
  }
  asPosition(model) {
  - return model.positionForOffset(this.offset, this.atEnd);
  + return model.positionForOffset(this.offset, this.atNodeEnd);
  }
  }
  