Optimization for autoload misses, refs #964

src/Composer/Autoload/ClassLoader.php

@@ -201,5 +201,7 @@ class ClassLoader
         if ($this->useIncludePath && $file = stream_resolve_include_path($classPath)) {
             return $file;
         }
+
+        $this->classMap[$class] = false;
     }
 }