code: 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
      ];
    }
  }
docs: 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.