Sets a threshold of five continuations determining if the old
if-else or the new igoto-based implementation will be used.
Informal benchmarking on x64 and arm_x64 point towards the overhead
of the igoto-based impl. dropping off around this point.
Benchmarks of this CL (threshold=5) show drastic improvement in
Calls.IterableManualIterablePolymorphicManyYields of about ~35-65%
across {dart,dart-aot}-{ia32,x64,armv7hf,armv8}.
Bug: https://github.com/dart-lang/sdk/issues/37754
Change-Id: I6e113f1f98e9ab0f994cf93004227d616e9e4d07
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/133586
Commit-Queue: Clement Skau <cskau@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>
