transformGeneratorBody(tree, name) {
    let createGeneratorInstance =
        this.getRuntimeExpression('createGeneratorInstance');
    return this.transformCpsFunctionBody(tree, createGeneratorInstance, name);
  }
Transform a generator function body - removing yield 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.createGeneratorInstance(machineFunction);
}

@param {FunctionBody} tree
@param {IdentifierExpression} name
@return {FunctionBody}