[js-import-plugin] Incorrectly reporting dependency errors

If file A.js declares a dependency on var B, and B.js will declare B but isn't yet created, the plugin will correctly state that B cannot be resolved.

If B.js is then created declaring B then the plugin is fine.

However if a change is subsequently made to A.js, it still complains that B does not exist.

At this time if the project is cleanly built, or built from the command line (where a scan of all js files is performed), then all is well.