Use sync iterator in coro-prime-sieve for Dart

[blaugold]

core-prime-sieve/1.dart uses async iteration which has considerable overhead.

[hanabi1224]

Sync generators are not allowed in this problem, plz refer to #279 (comment)

[blaugold]

Ah, sorry, got it.

Just as a heads-up: In Dart 2.18.0 the async implementation has been rewritten. The sort of code used in coro-prime-sieve essentially is lowered into synchronous code. This leads to stack-overflows for higher n values (>1000).

One way to fix this is to replace yield i; in filter with yield await Future.microtask(() => i);