I think when people saw that Dart was from Gilad Bracha and Lars Bak there was an expectation that Dart was going to be a grand synthesis: a blazingly-fast NewSpeak-with-curly-brackets. It’s very much not such a language. It doesn’t seem, academically, vastly innovative because it doesn’t add much. But, in truth, optional types are a radical design decision in that they take away runtime aspects that a lot of mainstream programmers expect. (Of course, this raises the question of how to define the “mainstream”…)
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Pros and Cons of Mandatory Typing In Descending Order of Importance (per Gilad Bracha):
Pros:
- machine-checkable documentation
- types provide conceptual framework
- early error detection
- performance advantages
Cons:
- expressiveness curtailed
- imposes workflow
- brittleness
…
Having said that, I attended a lecture in which someone, perhaps from Adobe, measured the performance impact of optional typing. Their conclusion, although admittedly done on the troublesome-ly small and artificial SunSpider benchmarks, was that the performance penalty of implicit types amounts to 40% (with a very large standard of deviation). That “feels” about right to me -- definitely significant but not the overwhelming performance benefit you might get from either parallelization or an algorithmic change.