framework/base/Theme.php
public function getUrl($url)
    {
        if (($baseUrl = $this->getBaseUrl()) !== null) {
            return $baseUrl . '/' . ltrim($url, '/');
        }

        throw new InvalidConfigException('The "baseUrl" property must be set.');
    }
public function getUrl($url)
    {
        if (($baseUrl = $this->getBaseUrl()) !== null) {
            return $baseUrl . '/' . ltrim($url, '/');
        }

        throw new InvalidConfigException('The "baseUrl" property must be set.');
    }
Converts a relative URL into an absolute URL using [[baseUrl]].
@param string $url the relative URL to be converted.
@return string the absolute URL
@throws InvalidConfigException if [[baseUrl]] is not set