(normal JS is faster on 3, matched on 1, and slower on 1).
Of course, the usual caveats apply regarding benchmarks and real-world performance, especially since those 5 benchmarks are fairly small and probably not representative of large real-world codebases. I'm not aware of any good benchmarks for Dart on that (there is a Box2D comparison, but the latest update isn't recent).
> (normal JS is faster on 3, matched on 1, and slower on 1).
And I would be really skeptical about that 1 where they say dart2js is faster.
They removed a layer of indirection in DeltaBlue when they ported it to Dart (not due to any Dart language feature, just for no reason), then crowed about how dart2js made DeltaBlue faster than the original. Despite being made aware of it years ago, they never fixed this! But the JavaScript VMs got better enough to be faster even though running code with more indirection than the dart2js version based on the optimized port.
So when the one benchmark dart2js is faster on is one they say they rewrote because a direct comparison would be "unfair" it sets off alarms.
Compare the source codes for yourself. It should be obvious that OrderedCollection exists in the js version, left over from the original Smalltalk, and not in Dart. Dart devs don't like criticism.
Actually on most of the official benchmarks, dart2js output is slower than normal JS,
https://www.dartlang.org/performance/
(normal JS is faster on 3, matched on 1, and slower on 1).
Of course, the usual caveats apply regarding benchmarks and real-world performance, especially since those 5 benchmarks are fairly small and probably not representative of large real-world codebases. I'm not aware of any good benchmarks for Dart on that (there is a Box2D comparison, but the latest update isn't recent).