startup: use `.hasOwnProperty` instead of `in`

Benchmarks show that `.hasOwnProperty` is faster than `in` in V8. It
makes startup ~0.5 ms faster on my computer.

src/node.js

@@ -491,7 +491,7 @@
   }
 
   NativeModule.exists = function(id) {
-    return (id in NativeModule._source);
+    return NativeModule._source.hasOwnProperty(id);
   }
 
   NativeModule.getSource = function(id) {