Add microbenchmark for private properties

bench/snippets/private.mjs

@@ -0,0 +1,80 @@
+import { bench, run } from "../node_modules/mitata/src/cli.mjs";
+// This is a benchmark of the performance impact of using private properties.
+
+bench("Polyfillprivate", () => {
+  "use strict";
+  var __classPrivateFieldGet =
+    (this && this.__classPrivateFieldGet) ||
+    function (receiver, state, kind, f) {
+      if (kind === "a" && !f) throw new TypeError("Private accessor was defined without a getter");
+      if (typeof state === "function" ? receiver !== state || !f : !state.has(receiver))
+        throw new TypeError("Cannot read private member from an object whose class did not declare it");
+      return kind === "m" ? f : kind === "a" ? f.call(receiver) : f ? f.value : state.get(receiver);
+    };
+  var __classPrivateFieldSet =
+    (this && this.__classPrivateFieldSet) ||
+    function (receiver, state, value, kind, f) {
+      if (kind === "m") throw new TypeError("Private method is not writable");
+      if (kind === "a" && !f) throw new TypeError("Private accessor was defined without a setter");
+      if (typeof state === "function" ? receiver !== state || !f : !state.has(receiver))
+        throw new TypeError("Cannot write private member to an object whose class did not declare it");
+      return kind === "a" ? f.call(receiver, value) : f ? (f.value = value) : state.set(receiver, value), value;
+    };
+  var _Foo_state, _Foo_inc;
+  class Foo {
+    constructor() {
+      _Foo_state.set(this, 1);
+      _Foo_inc.set(this, 13);
+    }
+    run() {
+      let n = 1000000;
+      while (n-- > 0) {
+        __classPrivateFieldSet(
+          this,
+          _Foo_state,
+          __classPrivateFieldGet(this, _Foo_state, "f") + __classPrivateFieldGet(this, _Foo_inc, "f"),
+          "f",
+        );
+      }
+      return n;
+    }
+  }
+  (_Foo_state = new WeakMap()), (_Foo_inc = new WeakMap());
+  new Foo().run();
+});
+
+bench("NativePrivates", () => {
+  class Foo {
+    #state = 1;
+    #inc = 13;
+
+    run() {
+      let n = 1000000;
+      while (n-- > 0) {
+        this.#state += this.#inc;
+      }
+      return n;
+    }
+  }
+
+  new Foo().run();
+});
+
+bench("ConventionalPrivates", () => {
+  class Foo {
+    _state = 1;
+    _inc = 13;
+
+    run() {
+      let n = 1000000;
+      while (n-- > 0) {
+        this._state += this._inc;
+      }
+      return n;
+    }
+  }
+
+  new Foo().run();
+});
+
+await run();