electron flow type definitions

Summary: Adding a flowtype library definition for electron 3 and fixing related type errors

Reviewed By: passy

Differential Revision: D9124758

fbshipit-source-id: e09cb5b05ba952e7f95f68f9043edc586f81ae83

src/ui/components/Button.js

@@ -281,7 +281,8 @@ class Button extends styled.StylableComponent<
         position.x = parseInt(left, 10);
         position.y = parseInt(bottom + 6, 10);
       }
-      menu.popup(electron.remote.getCurrentWindow(), {
+      menu.popup({
+        window: electron.remote.getCurrentWindow(),
         async: true,
         ...position,
       });