[tests] Fallback for fs.promises on node 6

packages/babel-core/test/config-chain.js

@@ -4,6 +4,20 @@ import path from "path";
 import escapeRegExp from "lodash/escapeRegExp";
 import { loadOptions as loadOptionsOrig } from "../lib";
 
+// TODO: In Babel 8, we can directly uses fs.promises which is supported by
+// node 8+
+const pfs =
+  fs.promises ??
+  new Proxy(fs, {
+    get: (target, name) => (...args) =>
+      new Promise((resolve, reject) =>
+        target[name](...args, (error, result) => {
+          if (error) reject(error);
+          else resolve(result);
+        }),
+      ),
+  });
+
 function fixture(...args) {
   return path.join(__dirname, "fixtures", "config", ...args);
 }
@@ -26,10 +40,10 @@ function pairs(items) {
 }
 
 async function getTemp(name) {
-  const cwd = await fs.promises.mkdtemp(os.tmpdir() + path.sep + name);
+  const cwd = await pfs.mkdtemp(os.tmpdir() + path.sep + name);
   const tmp = name => path.join(cwd, name);
   const config = name =>
-    fs.promises.copyFile(fixture("config-files-templates", name), tmp(name));
+    pfs.copyFile(fixture("config-files-templates", name), tmp(name));
   return { cwd, tmp, config };
 }