Highlighting for ES2015+ syntax is unreadable

[quickdudley]

This example code if saved as a file with the .js suffix and opened with Yi will be rendered with the JavaScript syntax highlighter. Since it uses a superset of the JavaScript syntax supported by the hightlighter: there are many parts which are hightlighted red but are actually accepted by node.js. This would be a minor nuisance: but there are sections which are red on red so those sections are completely unreadable.