Revert change to move `super` to top of constructor

It turns out that there is an implicit requirement that `Host` call its
parent constructor _after_ the `.annotator-frame` element has been
created and added to the DOM. This needs refactoring to avoid the hidden
dependency, but I'm leaving that for later to focus on the CoffeeScript
=> JS conversion here.

src/annotator/host.js

@@ -6,8 +6,6 @@ const Guest = require('./guest');
 
 export default class Host extends Guest {
   constructor(element, config) {
-    super(element, config);
-
     // Some config settings are not JSON-stringifiable (e.g. JavaScript
     // functions) and will be omitted when the config is JSON-stringified.
     // Add a JSON-stringifiable option for each of these so that the sidebar can
@@ -65,20 +63,32 @@ export default class Host extends Guest {
       );
     }
 
+    let externalFrame;
+    let frame;
+
     if (externalContainer) {
-      this.externalFrame = $(externalContainer);
+      externalFrame = $(externalContainer);
     } else {
-      this.frame = $('<div></div>')
+      frame = $('<div></div>')
         .css('display', 'none')
         .addClass('annotator-frame annotator-outer');
 
       if (config.theme === 'clean') {
-        this.frame.addClass('annotator-frame--drop-shadow-enabled');
+        frame.addClass('annotator-frame--drop-shadow-enabled');
       }
 
-      this.frame.appendTo(element);
+      frame.appendTo(element);
     }
 
+    // FIXME: We have to call the parent constructor here instead of at the top
+    // of the function because it triggers plugin construction and the BucketBar
+    // plugin constructor in turn assumes that the `.annotator-frame` element is
+    // already in the DOM.
+    super(element, config);
+
+    this.externalFrame = externalFrame;
+    this.frame = frame;
+
     app.appendTo(this.frame || this.externalFrame);
 
     this.on('panelReady', () => {