Borrowed from https://github.com/laravel/valet/pull/47 with fixes so that it runs from Magento root instead of pub directory. Put this in ~/.valet/Drivers/Magento2ValetDriver.php

Magento2ValetDriver.php

<?php

class Magento2ValetDriver extends ValetDriver
{

    /**
     * Holds the MAGE_MODE from app/etc/config.php or $ENV
     *
     * @var string
     */
    private $mageMode;

    /**
     * Determine if the driver serves the request.
     *
     * @param  string $sitePath
     * @param  string $siteName
     * @param  string $uri
     * @return boolean
     */
    public function serves($sitePath, $siteName, $uri)
    {
        return file_exists($sitePath . '/bin/magento') && file_exists($sitePath . '/pub/index.php');
    }

    /**
     * Determine if the incoming request is for a static file.
     *
     * @param  string $sitePath
     * @param  string $siteName
     * @param  string $uri
     * @return string|false
     */
    public function isStaticFile($sitePath, $siteName, $uri)
    {
        $this->checkMageMode($sitePath);

        $uri = $this->handleForVersions($uri);
        $route = parse_url(substr($uri, 1))['path'];

        $pub = '';
        if ('developer' === $this->mageMode) {
            $pub = 'pub/';
        }

        if (!$this->isPubDirectory($sitePath, $route, $pub)) {
            return false;
        }

        $magentoPackagePubDir = $sitePath;
        if ('developer' !== $this->mageMode) {
            $magentoPackagePubDir .= '/pub';
        }

        $file = $magentoPackagePubDir . '/' . $route;

        if (file_exists($file)) {
            return $magentoPackagePubDir . $uri;
        }

        if (strpos($route, $pub . 'static/') === 0) {
            $route = preg_replace('#' . $pub . 'static/#', '', $route, 1);
            $_GET['resource'] = $route;
            include $magentoPackagePubDir . '/' . $pub . 'static.php';
            exit;
        }

        if (strpos($route, $pub . 'media/') === 0) {
            include $magentoPackagePubDir . '/' . $pub . 'get.php';
            exit;
        }

        return false;
    }

    /**
     * Rewrite URLs that look like "versions12345/" to remove
     * the versions12345/ part
     *
     * @param  string $route
     */
    private function handleForVersions($route)
    {
        return preg_replace('/version\d*\//', '', $route);
    }

    /**
     * Determine the current MAGE_MODE
     *
     * @param  string $sitePath
     */
    private function checkMageMode($sitePath)
    {
        if (null !== $this->mageMode) {
            // We have already figure out mode, no need to check it again
            return;
        }
        if (!file_exists($sitePath . '/index.php')) {
            $this->mageMode = 'production'; // Can't use developer mode without index.php in project root
            return;
        }
        $mageConfig = [];
        if (file_exists($sitePath . '/app/etc/env.php')) {
            $mageConfig = require $sitePath . '/app/etc/env.php';
        }
        if (array_key_exists('MAGE_MODE', $mageConfig)) {
            $this->mageMode = $mageConfig['MAGE_MODE'];
        }
    }

    /**
     * Checks to see if route is referencing any directory inside pub. This is a dynamic check so that if any new
     * directories are added to pub this driver will not need to be updated.
     *
     * @param string $sitePath
     * @param string $route
     * @param string $pub
     * @return bool
     */
    private function isPubDirectory($sitePath, $route, $pub = '')
    {
        $sitePath .= '/pub/';
        $dirs = glob($sitePath . '*', GLOB_ONLYDIR);

        $dirs = str_replace($sitePath, '', $dirs);
        foreach ($dirs as $dir) {
            if (strpos($route, $pub . $dir . '/') === 0) {
                return true;
            }
        }
        return false;
    }

    /**
     * Get the fully resolved path to the application's front controller.
     *
     * @param  string $sitePath
     * @param  string $siteName
     * @param  string $uri
     * @return string
     */
    public function frontControllerPath($sitePath, $siteName, $uri)
    {
        $this->checkMageMode($sitePath);

        if ('developer' === $this->mageMode) {
            return $sitePath . '/index.php';
        }
        return $sitePath . '/pub/index.php';
    }
}