[vm] Make async lowering no longer use optional parameters

This CL

  * Makes :async_op only have two parameters: We take advantage
    of the fact that errors not only have exception != null but
    also stacktrace != null.
  * Makes :async_op have no optional parameters. This reduces
    code size significantly.
  * Removes unused parameter in _awaitHelper calls
  * Wrap the then callback instead of the error callback. (needed
    to make optional parameters required)

This results in 2% code savings on a big g3 app.
The size of :async_op shrinks on average by 11%

TEST=ci

Change-Id: I38d5fba4ebebc780b48dac5aa6a250d2c7952bfd
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/233362
Reviewed-by: Slava Egorov <vegorov@google.com>
Reviewed-by: Alexander Markov <alexmarkov@google.com>
Commit-Queue: Martin Kustermann <kustermann@google.com>