use promises to wait for kernelspecs on notebook load

should avoid race condition that causes attempt to load kernelspec
prior to loading the kernelspec list

IPython/html/static/notebook/js/kernelselector.js

@@ -9,6 +9,7 @@ define([
     "use strict";
     
     var KernelSelector = function(selector, notebook) {
+        var that = this;
         this.selector = selector;
         this.notebook = notebook;
         this.notebook.set_kernelselector(this);
@@ -22,6 +23,11 @@ define([
         this.bind_events();
         // Make the object globally available for user convenience & inspection
         IPython.kernelselector = this;
+        this._finish_load = null;
+        this.loaded = new Promise(function(resolve, reject) {
+            that._finish_load = resolve;
+        });
+        
         Object.seal(this);
     };
     
@@ -67,6 +73,8 @@ define([
                 .text(ks.spec.display_name));
             new_notebook_submenu.append(ks_submenu_entry);
         }
+        // trigger loaded promise
+        this._finish_load();
     };
     
     KernelSelector.prototype._spec_changed = function (event, ks) {

IPython/html/static/notebook/js/notebook.js

@@ -2077,11 +2077,16 @@ define([
      * @param {string} notebook_path - A notebook to load
      */
     Notebook.prototype.load_notebook = function (notebook_path) {
+        var that = this;
         this.notebook_path = notebook_path;
         this.notebook_name = utils.url_path_split(this.notebook_path)[1];
         this.events.trigger('notebook_loading.Notebook');
         this.contents.get(notebook_path, {type: 'notebook'}).then(
-            $.proxy(this.load_notebook_success, this),
+            function (data) {
+                that.kernel_selector.loaded.then(
+                    that.load_notebook_success(data)
+                );
+            },
             $.proxy(this.load_notebook_error, this)
         );
     };