Could you clarify what "smacks" of a language that isn't very mature? There are libraries written in any language that are not tuned for performance.
The goal of this post is to show that poor performance of a single library should not be interpreted as poor performance of a language feature that this library uses... Instead you must rigorously evaluate performance issues. (And also provide feedback to language implementors)
It is true that builtin libraries could have provided a better JSON deserialization support... So if your definition of an immature general-purpose language depends on whether or not there is a fast JSON deserialize built right into the core libraries - then Dart certainly does not pass.
However one can also define a general purpose programming language as a language in which programmer could efficiently implement all sorts of things, including but not limited to an efficient JSON deserializer - which seems to be the case for Dart.
> why would I choose Dart over Go?
I don't know. Why do you have to choose Dart over Go? You choose want you like most and what makes you most productive. For me it would be Dart for you it might be Go. Such is life. Not everybody has to write everything in the same language.
We can offer you to try Dart. You can refuse to try. You can also try it and then refuse to use it... Endless possibilities :)