tools(bench): rebootstrap (denoland#12281)

Enable deno devs to bench/profile/test JS code changes without doing a full --release rebuild.

Incremental release builds take ~4mn on M1s, often more on other machines ...

tools/bench/README.md

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+## Re-bootstrapping
+
+Re-bootstrapping allows deno devs to bench/profile/test JS-side changes without
+doing a full `cargo build --release --bin deno` which takes roughly ~4mn on M1s
+more on other machines which significantly slows down iteration &
+experimentation.
+
+## Example
+
+```js
+import { benchSync, rebootstrap } from "./tools/bench/mod.js";
+
+const bootstrap = rebootstrap([
+  "webidl",
+  "console",
+  "url",
+  "web",
+  "fetch",
+]);
+
+benchSync("resp_w_h", 1e6, () =>
+  new bootstrap.fetch.Response("yolo", {
+    status: 200,
+    headers: {
+      server: "deno",
+      "content-type": "text/plain",
+    },
+  }));
+```
+
+This code can then benched and profiled (using Chrome's DevTools) similar to
+regular userland code and the original source files appear in the DevTools as
+you would expect.

tools/bench/mod.js

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+export * from "./rebench.js";
+export * from "./rebootstrap.js";

tools/bench/rebench.js

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+export function benchSync(name, n, innerLoop) {
+  const t1 = Date.now();
+  for (let i = 0; i < n; i++) {
+    innerLoop(i);
+  }
+  const t2 = Date.now();
+  console.log(benchStats(name, n, t1, t2));
+}
+
+export async function benchAsync(name, n, innerLoop) {
+  const t1 = Date.now();
+  for (let i = 0; i < n; i++) {
+    await innerLoop(i);
+  }
+  const t2 = Date.now();
+  console.log(benchStats(name, n, t1, t2));
+}
+
+function benchStats(name, n, t1, t2) {
+  const dt = (t2 - t1) / 1e3;
+  const r = n / dt;
+  const ns = Math.floor(dt / n * 1e9);
+  return `${name}:${" ".repeat(20 - name.length)}\t` +
+    `n = ${n}, dt = ${dt.toFixed(3)}s, r = ${r.toFixed(0)}/s, t = ${ns}ns/op`;
+}

tools/bench/rebootstrap.js

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+import { dirname, fromFileUrl, join } from "https://deno.land/std/path/mod.ts";
+import { expandGlobSync } from "https://deno.land/std/fs/mod.ts";
+
+const ROOT_DIR = join(dirname(fromFileUrl(import.meta.url)), "..", "..");
+
+export function rebootstrap(exts) {
+  [
+    "core/00_primordials.js",
+    ...exts.map((e) => `ext/${e}/*.js`),
+  ]
+    .map((pattern) => join(ROOT_DIR, pattern))
+    .map((pattern) => [...expandGlobSync(pattern)])
+    .flat()
+    .map((entry) => entry.path)
+    .forEach((file) => {
+      Deno.core.evalContext(Deno.readTextFileSync(file), file);
+    });
+  const bootstrap = globalThis.__bootstrap;
+  delete globalThis.__bootstrap;
+  // Patch dispatchEvent so we don't crash when MainWorker exits via:
+  // `window.dispatchEvent(new Event('unload'))`
+  // which fails since symbols are mangled during rebootstrap
+  globalThis.dispatchEvent = () => {};
+  return bootstrap;
+}