transformAsyncBody(tree) {
    let asyncWrap = this.getRuntimeExpression('asyncWrap');
    return this.transformCpsFunctionBody(tree, asyncWrap);
  }
Transform an async function body - removing async statements.
The transformation is in two stages. First the statements are converted into a single
state machine by merging state machines via a bottom up traversal.

Then the final state machine is converted into the following code:

{
  machine variables
  return $traceurRuntime.asyncWrap(machineFunction);
}
@param {FunctionBody} tree
@return {FunctionBody}