function arrow(params, body) {
	return {
		type: 'ArrowFunctionExpression',
		params,
		body,
		expression: body.type !== 'BlockStatement',
		generator: false,
		async: false,
		metadata: /** @type {any} */ (null) // should not be used by codegen
	};
}
@param {Array<ESTree.Pattern>} params
@param {ESTree.BlockStatement | ESTree.Expression} body
@returns {ESTree.ArrowFunctionExpression}