[Kernel] Remove the syncAsync flag

This flag was always true so we can remove it and specialize the
remaining code that used it.  The only code that used it was in the
Dart VM's async transformation.

Specializing that code led to some unused fields in the async helper
class which were removed.  This triggered me to look at core types
which seems to have a lot of unused stuff.

Change-Id: I7da800ca17ac05478149f4c90337a38d2c2b1459
Reviewed-on: https://dart-review.googlesource.com/c/87960
Reviewed-by: Peter von der Ahé <ahe@google.com>
Commit-Queue: Kevin Millikin <kmillikin@google.com>