Improve @babel/runtime esm stability (#12883)

test/runtime-integration/bundlers.cjs

@@ -0,0 +1,48 @@
+const cp = require("child_process");
+const path = require("path");
+const fs = require("fs");
+
+for (const absolute of [false, true]) {
+  const output = absolute ? "output-absolute.js" : "output.js";
+  const title = absolute ? "(absolute runtime)" : "";
+
+  const webpack = absolute
+    ? "webpack --config webpack.absolute.config.js"
+    : "webpack";
+  const rollup = absolute ? "rollup -c rollup.absolute.config.js" : "rollup -c";
+
+  // TODO: This never worked in any Babel version
+  if (!absolute) {
+    test(`Webpack 5 ${title}`, webpack, "webpack-5", output, true);
+  }
+  test(`Webpack 4 ${title}`, webpack, "webpack-4", output);
+  test(`Webpack 3 ${title}`, webpack, "webpack-3", output);
+  test(`Rollup ${title}`, rollup, "rollup", output);
+}
+
+function test(name, command, directory, output, first) {
+  console.log(`Building with ${name}`);
+  cp.execSync(`yarn ${command}`, {
+    cwd: path.join(__dirname, directory),
+    encoding: "utf8",
+  });
+  console.log(`Testing the ${name} bundle`);
+  const out = cp.execSync(`node ${output}`, {
+    cwd: path.join(__dirname, directory),
+    encoding: "utf8",
+  });
+
+  const expectedPath = path.join(__dirname, "expected-bundler.txt");
+  let expected = fs.readFileSync(expectedPath, "utf8");
+
+  if (expected === out) {
+    console.log("OK");
+  } else if (first && process.env.OVERWRITE) {
+    fs.writeFileSync(expectedPath, out);
+    expected = out;
+    console.log("UPDATED");
+  } else {
+    console.error("FAILED\n");
+    console.error(out);
+  }
+}