Dispose of toolbox when workspace is disposed.

core/toolbox.js

@@ -139,6 +139,15 @@ Blockly.Toolbox.prototype.init = function() {
   this.position();
 };
 
+/**
+ * Dispose of this toolbox.
+ */
+Blockly.Toolbox.prototype.dispose = function() {
+  this.flyout_.dispose();
+  this.tree_.dispose();
+  goog.dom.removeNode(this.HtmlDiv);
+};
+
 /**
  * Move the toolbox to the edge.
  */