Make `Adder#showAt` a method

src/annotator/adder.js

@@ -240,6 +240,7 @@ class Adder {
 
       return {top, left, arrowDirection};
     };
+  }
 
   /**
    * Show the adder at the given position and with the arrow pointing in
@@ -248,7 +249,7 @@ class Adder {
    * @param {number} left - Horizontal offset from left edge of viewport.
    * @param {number} top - Vertical offset from top edge of viewport.
    */
-    this.showAt = (left, top, arrowDirection) => {
+  showAt(left, top, arrowDirection) {
     this.element.className = classnames({
       'annotator-adder': true,
       'annotator-adder--arrow-down': arrowDirection === ARROW_POINTING_DOWN,
@@ -282,7 +283,6 @@ class Adder {
     this._enterTimeout = setTimeout(() => {
       this.element.className += ' is-active';
     }, 1);
-    };
   }
 }