Even the original, untyped, Dart was designed to be easier to optimize that JavaScript (e.g. it's class based, not prototype based, so the runtime doesn't have t work as much to optimize object access).

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...).