Bug 1261505 - Use consistent syntax for prototype methods; r=jsantell

All the other methods use the style

  {
    myMethod() { ... }
  }

while these two methods use

  {
    myMethod: function () { ... }
  }

For consistency, we should use the common style here.

MozReview-Commit-ID: 5rg8uPoULyN

devtools/client/shared/components/tree.js

@@ -369,7 +369,7 @@ const Tree = module.exports = createClass({
    *
    * @param {Object} item
    */
-  _focus: function (item) {
+  _focus(item) {
     if (this.props.onFocus) {
       this.props.onFocus(item);
     }
@@ -378,7 +378,7 @@ const Tree = module.exports = createClass({
   /**
    * Sets the state to have no focused item.
    */
-  _onBlur: function () {
+  _onBlur() {
     this._focus(undefined);
   },