When I look at Dart vs JS (JavaScript), I see a lot of parallels to Java/C#. The Java language stagnated for a long time under Sun. The language was usable only through the heroic efforts of tool vendors (Eclipse, IntelliJ, etc.) Even with these tools, Java is just barely good enough.

C# is technically a superior language to Java. Combined with an excellent IDE (VS), the .NET platform is probably technically superior to the Java platform. However, .NET did not destroy Java. And it's because Java is superior to .NET in many non-technical areas (such as a better open source community). And other solutions appeared to help the Java platform with the language stagnation issue (Scala, Clojure, Groovy, etc.). Now Java itself is somewhat catching up (Java 8) and the JVM platform remains a powerful and thriving community.

The situation with Dart and JS seems to be similar. The JS language has stagnated for a long time. Google is trying to be like MS with C# and created Dart to fully replace JS. And Dart is probably technically superior to JS. But just like Java vs C#, I don't think technical superiority will be enough.

The JS platform is addressing the language issue with CoffeeScript, TypeScript, and others, just like Scala and Clojure appeared for the JVM. And JS itself is improving slowly (ECMAScript 6) just like Java 8. And the JS open source community is very very strong (all the web related libraries and frameworks and now additional strength from Node.JS) and it's unlikely Dart will have a comparable community. I think that Dart may achieve some success like .NET/C# did. However, I don't think it will destroy JS like Google hopes it will. The JS platform is just like the JVM platform. Has so many flaws, but is good enough.