Optimize Dart version

[mraleph]

Main changes:

• Dart's AOT compiler looses track of concrete types when containers-containing-other-containers are involved, e.g. Map<K, List<T>> or List<List<T>>. This makes operations wither nested containers slower. To work-around this problem you can simply wrap a nested container into some unique class - this helps AOT compiler to realise what comes from the container.
• Iterable.fillRange is slow - manually inlining it helps (I will file a bug against Dart to fix this).
• for-in is better than forEach when concrete types are properly known to the compiler (see the first item)
• Rewrote computation of top 5 to avoid unnecessary allocations

[mraleph]

These changes improve Dart AOT performance from 183ms to 100ms on my machine (measured on HEAD of Dart SDK - the upcoming release has a bunch of improvements which make for-in perform better in AOT mode).

[jinyus]

Thanks for the PR. Very insightful. Drops from 70ms -> 55 on my machine.