make babel injectable in babel-register

packages/babel-register/src/node.js

@@ -2,7 +2,7 @@ import deepClone from "lodash/cloneDeep";
 import sourceMapSupport from "source-map-support";
 import * as registerCache from "./cache";
 import escapeRegExp from "lodash/escapeRegExp";
-import * as babel from "@babel/core";
+import * as babelCore from "@babel/core";
 import { OptionManager, DEFAULT_EXTENSIONS } from "@babel/core";
 import { addHook } from "pirates";
 import fs from "fs";
@@ -10,6 +10,7 @@ import path from "path";
 
 const maps = {};
 const transformOpts = {};
+let babel = babelCore;
 let piratesRevert = null;
 
 function installSourceMapSupport() {
@@ -100,6 +101,9 @@ register({
 export default function register(opts?: Object = {}) {
   // Clone to avoid mutating the arguments object with the 'delete's below.
   opts = Object.assign({}, opts);
+
+  if (opts.babel) babel = opts.babel;
+
   if (opts.extensions) hookExtensions(opts.extensions);
 
   if (opts.cache === false && cache) {
@@ -110,6 +114,7 @@ export default function register(opts?: Object = {}) {
     cache = registerCache.get();
   }
 
+  delete opts.babel;
   delete opts.extensions;
   delete opts.cache;
 

packages/babel-register/test/index.js

@@ -103,4 +103,20 @@ describe("@babel/register", function() {
     chai.expect((gen_error = require(GEN_ERROR))).to.be.ok;
     chai.expect(gen_error()).to.match(/gen_error\.js:2:86/);
   });
+
+  it("uses custom babel when requested", () => {
+    setupRegister({
+      babel: {
+        transform() {
+          return { code: "module.exports = 'hello';" };
+        },
+        getEnv() {
+          return {};
+        },
+        version: "custom",
+      },
+    });
+
+    chai.expect(require(DATA_ES2015)).to.equal("hello");
+  });
 });