Shell out to HHVM to execute the autoload generator

This is required for nightlies and the next release, given that composer
(and the plugin) can not be executed with HHVM.

src/ComposerPlugin.php

@@ -34,7 +34,7 @@ final class ComposerPlugin
   /** Initialize members */
   public function activate(Composer $composer, IOInterface $io) {
     $this->io = $io;
-    $vendor = $composer->getConfig()->get('vendor-dir', '/');
+    $vendor = $composer->getConfig()->get('vendor-dir');
 
     $this->vendor = $vendor;
     $this->root = dirname($vendor);
@@ -57,34 +57,7 @@ public static function getSubscribedEvents() {
    * Here we update our autoload map.
    */
   public function onPostAutoloadDump(Event $event) {
-    $this->debugMessage("Loading composer autoload");
-    require_once($this->vendor.'/autoload.php');
-
-    $this->debugMessage("Parsing tree");
-    $importer = new RootImporter(
-      $this->root,
-      $event->isDevMode()
-        ? IncludedRoots::DEV_AND_PROD
-        : IncludedRoots::PROD_ONLY
-    );
-
-    $handler = $event->isDevMode()
-      ? $importer->getConfig()['devFailureHandler']
-      : $importer->getConfig()['failureHandler'];
-
-    $this->debugMessage("Writing hh_autoload.php");
-    (new Writer())
-      ->setBuilder($importer)
-      ->setRoot($this->root)
-      ->setRelativeAutoloadRoot($importer->getConfig()['relativeAutoloadRoot'])
-      ->setFailureHandler($handler)
-      ->setIsDev($event->isDevMode())
-      ->writeToFile($this->vendor.'/hh_autoload.hh');
-  }
-
-  private function debugMessage(\HH\string $message) {
-    if ($this->io->isDebug()) {
-      $this->io->write('hhvm-autoload: '.$message);
-    }
+    $args = $event->isDevMode() ? '' : ' --no-dev';
+    shell_exec('hhvm '.escapeshellarg($this->vendor.'/bin/hh-autoload').$args);
   }
 }