repl: Keep the built-in modules non-enumerable

Make sure that the built-in modules in the repl stay non-enumerable.
Previously, they would pop up as enumerable properties of the global
object after having been accessed for the first time.

lib/internal/module.js

@@ -59,21 +59,36 @@ exports.builtinLibs = ['assert', 'buffer', 'child_process', 'cluster',
 function addBuiltinLibsToObject(object) {
   // Make built-in modules available directly (loaded lazily).
   exports.builtinLibs.forEach((name) => {
+    // Goals of this mechanism are:
+    // - Lazy loading of built-in modules
+    // - Having all built-in modules available as non-enumerable properties
+    // - Allowing the user to re-assign these variables as if there were no
+    //   pre-existing globals with the same name.
+
+    const setReal = (val) => {
+      // Deleting the property before re-assigning it disables the
+      // getter/setter mechanism.
+      delete object[name];
+      object[name] = val;
+    };
+
     Object.defineProperty(object, name, {
       get: () => {
         const lib = require(name);
-        // This implicitly invokes the setter, so that this getter is only
-        // invoked at most once and does not overwrite anything.
-        object[name] = lib;
-        return lib;
-      },
-      // Allow the creation of other globals with this name.
-      set: (val) => {
-        // Deleting the property before re-assigning it disables the
-        // getter/setter mechanism.
+
+        // Disable the current getter/setter and set up a new
+        // non-enumerable property.
         delete object[name];
-        object[name] = val;
+        Object.defineProperty(object, name, {
+          get: () => lib,
+          set: setReal,
+          configurable: true,
+          enumerable: false
+        });
+
+        return lib;
       },
+      set: setReal,
       configurable: true,
       enumerable: false
     });