For a newer web programmer learning a toolset to get things done quick is much more important than jumping on any one bandwagon. If you can't build any complex systems with what you have learned then jumping languages is not going to help you. Use what you know and build great software and when you move onto the next project you'll know where your language isn't the greatest and it's worth shopping around to know what would do it better.

Javascript is going to be around a long time, you aren't going to just wake up tomorrow and see Dart paving over everything Javascript was doing. Sticking to the beaten path when starting out, especially when the path you've chosen is ubiquitous, is not a bad way to go about things.

Until Dart is natively supported in Browsers (which might take several years, at the very least), it is going to be cross compiled to Javascript before being shipped to the end -user.

Knowing and understanding javascript and it's intricacies will be invaluable to writing and debugging good Dart code.