function build_fallback(expression, fallback) {
	if (is_simple_expression(fallback)) {
		return b.call('$.fallback', expression, fallback);
	}

	if (fallback.type === 'AwaitExpression' && is_simple_expression(fallback.argument)) {
		return b.await(b.call('$.fallback', expression, fallback.argument));
	}

	return is_expression_async(fallback)
		? b.await(b.call('$.fallback', expression, b.thunk(fallback, true), b.true))
		: b.call('$.fallback', expression, b.thunk(fallback), b.true);
}
@param {ESTree.Expression} expression
@param {ESTree.Expression} fallback