bug in async/scala-2.12.x
async/types generate in state-machine next chunk of code:
val result: scala.concurrent.Promise[Int] = Promise.apply[Int]();
<stable> <accessor> def result: scala.concurrent.Promise[Int] = stateMachine$macro$1041.this.result;
val execContext: scala.concurrent.ExecutionContext = ..
<stable> <accessor> def execContext: scala.concurrent.Promise[Int] = stateMachine$macro$1041.this.execContext;
when we attempt untype/type code again, it is not compiled.
So, we need to remove result and execContext DefDefs
short representation of tree, suitable for show in error messages.