add comment to monitor block listener filtering of events

src/virtual-machine.js

@@ -353,6 +353,8 @@ class VirtualMachine extends EventEmitter {
      * @param {!Blockly.Event} e Any Blockly event.
      */
     monitorBlockListener (e) {
+        // Filter events by type, since monitor blocks only need to listen to these events.
+        // Monitor blocks shouldn't be destroyed when flyout blocks are deleted.
         if (['create', 'change'].indexOf(e.type) !== -1) {
             this.runtime.monitorBlocks.blocklyListen(e, this.runtime);
         }