benchmark: rest operator benchmark

Benchmark comparing `util._extend()`, `Object.assign()`,
and the rest operator for object assignment.

`util._extend()` still wins currently.

benchmark/es/rest-assign.js

@@ -0,0 +1,47 @@
+'use strict';
+
+const common = require('../common.js');
+const assert = require('assert');
+const util = require('util');
+
+const bench = common.createBenchmark(main, {
+  method: ['rest', 'assign', '_extend'],
+  count: [5, 10, 20],
+  millions: [1]
+});
+
+function main({ millions, context, count, rest, method }) {
+  const n = millions * 1e6;
+
+  const src = {};
+  for (let n = 0; n < count; n++)
+    src[`p${n}`] = n;
+
+  let obj;
+  let i;
+
+  switch (method) {
+    case '':
+      // Empty string falls through to next line as default, mostly for tests.
+    case 'extend':
+      bench.start();
+      for (i = 0; i < n; i++)
+        obj = util._extend({}, src);
+      bench.end(n);
+      break;
+    case 'assign':
+      bench.start();
+      for (i = 0; i < n; i++)
+        obj = Object.assign({}, src);
+      bench.end(n);
+      break;
+    case 'rest':
+      bench.start();
+      for (i = 0; i < n; i++)
+        obj = { ...src };
+      bench.end(n);
+      break;
+    default:
+      throw new Error('Unexpected method');
+  }
+}