Recently Dart has added a strong mode (https://github.com/dart-archive/dev_compiler/blob/master/STR..., http://news.dartlang.org/2017/01/sound-dart-and-strong-mode....).
One of the reasons for strong mode is to enable better optimizations (https://www.dartlang.org/guides/language/sound-faq#why-stron...).
When JS performance threads come up I always mention that JS will never be as fast as Go/Java. Some people have a tantrum when I explain these issues. The sources I usually cite are dense and people just continue to downvote without reading them.
I think it's a work in progress, but main benefit at least at the time was faster startup.
Unlike the script snapshots, application snapshots also include CPU-specific native code in addition to the serialized token stream of the source code.
So, you don't just skip the parsing stage, you also skip the warmup phase. Your application starts instantly and it runs at full speed from the get-go.
On one hand, the website says that it's compiled to native code (so it can be same speed/faster than Java), but on the other hand, it's based on a soft-type language, which makes optimization difficult (even with V8, JS is still slower than native code).
Side question:
I understand that Dart was soft-typed because it was supposed to replace/compile to JS, but what advantage does soft-types have against "implied-types" (like auto or go's := ) combined with operator overloading (to allow string+int, for example)?