src/Composer/Installer/ProjectInstaller.php
public function install(InstalledRepositoryInterface $repo, PackageInterface $package)
    {
        $installPath = $this->installPath;
        if (file_exists($installPath) && !$this->filesystem->isDirEmpty($installPath)) {
            throw new \InvalidArgumentException("Project directory $installPath is not empty.");
        }
        if (!is_dir($installPath)) {
            mkdir($installPath, 0777, true);
        }
        $this->downloadManager->download($package, $installPath);
    }
public function install(InstalledRepositoryInterface $repo, PackageInterface $package)
    {
        $installPath = $this->installPath;
        if (file_exists($installPath) && !$this->filesystem->isDirEmpty($installPath)) {
            throw new \InvalidArgumentException("Project directory $installPath is not empty.");
        }
        if (!is_dir($installPath)) {
            mkdir($installPath, 0777, true);
        }
        $this->downloadManager->download($package, $installPath);
    }
{@inheritDoc}