Earlier quoted context omitted.
The main example is that Dart's basic numeric types can't be represented in JavaScript. Dart VM integers are bignums, while dart2js numbers are IEEE754 doubles.
That's for performance because I guess the majority of users would probably not notice the difference. They can be represented in JS, but not natively.
Re: Learn Dart in 15 Minutes
#41Yes, the decision to make numeric types incompatible between the VM and dart2js was due to speed concerns.