src/Composer/Installer/PluginInstaller.php
public function install(InstalledRepositoryInterface $repo, PackageInterface $package)
    {
        $extra = $package->getExtra();
        if (empty($extra['class'])) {
            throw new \UnexpectedValueException('Error while installing '.$package->getPrettyName().', composer-plugin packages should have a class defined in their extra key to be usable.');
        }

        parent::install($repo, $package);
        try {
            $this->composer->getPluginManager()->registerPackage($package, true);
        } catch (\Exception $e) {
            // Rollback installation
            $this->io->writeError('Plugin installation failed, rolling back');
            parent::uninstall($repo, $package);
            throw $e;
        }
    }
public function install(InstalledRepositoryInterface $repo, PackageInterface $package)
    {
        $extra = $package->getExtra();
        if (empty($extra['class'])) {
            throw new \UnexpectedValueException('Error while installing '.$package->getPrettyName().', composer-plugin packages should have a class defined in their extra key to be usable.');
        }

        parent::install($repo, $package);
        try {
            $this->composer->getPluginManager()->registerPackage($package, true);
        } catch (\Exception $e) {
            // Rollback installation
            $this->io->writeError('Plugin installation failed, rolling back');
            parent::uninstall($repo, $package);
            throw $e;
        }
    }
{@inheritDoc}