Gets the inner checks for a given return type.
This is used for async functions and generators.
Returns either null or an array of check nodes in the format: Y, R, N.
function getFunctionInnerChecks (context: ConversionContext, path: NodePath, returnType: NodePath): ? Array<?Node> {
  if (!path.node.async && !path.node.generator) {
    return;
  }
  if (!returnType.isGenericTypeAnnotation()) {
    return;
  }
  const returnTypeParameters = getTypeParameters(returnType);
  const id = returnType.get('id');
  const name = id.node.name;
  if (returnTypeParameters.length === 0) {
    // We're in an async or generator function but we don't have any type parameters.
    // We have to treat this as mixed.
    return [
      null,
      context.call('mixed'),
      null,
    ];
  }

  if (path.node.generator) {
    if (supportedIterableNames[name]) {
      const extra = [];
      for (const param of returnTypeParameters) {
        extra.push(convert(context, param));
      }
      return extra;
    }
  }
  else if (path.node.async) {
    // Make the return type a union with the promise resolution type.
    return [
      null,
      context.call(
        'union',
        convert(context, returnTypeParameters[0]),
        convert(context, returnType)
      ),
      null
    ];
  }
}