In my opinion, Swift, although great for shepherding more devs onto the Apple platform, fundamentally directs us further away from the fabled 'write once' dream. Introducing Dart to Android would just further this back-and-forth between proprietary platform holders.
And Microsoft, once again, is left utterly bemused, as it decided to focus away from proprietary languages and allow people to write native apps in JavaScript. But now people are excited about Swift, so is vendor-specific languages what people wanted all along?! How confusing.