CoffeeScript, TypeScript, GWT and every other language transpiling to JS gets hit with the compilation cost to first compile to JS (possibly waiting to emit source maps as well), after that point the browser can load and execute the compiled JavaScript.
When developing and running in the DartVM, Dart code can run natively inside the Dartium browser without needing to compile to JS first. Also Dart is declaratively structured and optimized for fast start-up times (e.g. opts for lazy loading) which means they can start JIT'ing and executing from the main(){} entry point (which is also the point where snapshots are saved at).
When developing and running in the DartVM, Dart code can run natively inside the Dartium browser without needing to compile to JS first. Also Dart is declaratively structured and optimized for fast start-up times (e.g. opts for lazy loading) which means they can start JIT'ing and executing from the main(){} entry point (which is also the point where snapshots are saved at).