Wrap extension menu function to provide context

When the extension system calls a function to retrieve extension menu
items, it now provides the ID of the current editing target. There's
also now a little bit of error-checking on the results.

src/extension-support/extension-manager.js

@@ -264,13 +264,36 @@ class ExtensionManager {
             // we can use it later when converting the menu for Scratch Blocks.
             if (typeof menus[item] === 'string') {
                 const serviceObject = dispatch.services[serviceName];
-                // TODO: Fix this to use dispatch.call when extensions are running in workers.
-                menus[item] = serviceObject[menus[item]].bind(serviceObject);
+                const menuName = menus[item];
+                menus[item] = this._getExtensionMenuItems.bind(this, serviceObject, menuName);
             }
         }
         return menus;
     }
 
+    /**
+     * Fetch the items for a particular extension menu, providing the target ID for context.
+     * @param {object} extensionObject - the extension object providing the menu.
+     * @param {string} menuName - the name of the menu function to call.
+     * @returns {Array} menu items ready for scratch-blocks.
+     * @private
+     */
+    _getExtensionMenuItems (extensionObject, menuName) {
+        // Fetch the items appropriate for the target currently being edited. This assumes that menus only
+        // collect items when opened by the user while editing a particular target.
+        const editingTarget = this.runtime.getEditingTarget();
+        const editingTargetID = editingTarget ? editingTarget.id : null;
+
+        // TODO: Fix this to use dispatch.call when extensions are running in workers.
+        const menuFunc = extensionObject[menuName];
+        const menuItems = menuFunc.call(extensionObject, editingTargetID);
+
+        if (!menuItems || menuItems.length < 1) {
+            throw new Error(`Extension menu returned no items: ${menuName}`);
+        }
+        return menuItems;
+    }
+
     /**
      * Apply defaults for optional block fields.
      * @param {string} serviceName - the name of the service hosting this extension block