Fix issue where babel was retained

packages/babel-register/src/node.js

@@ -10,7 +10,7 @@ import path from "path";
 
 const maps = {};
 const transformOpts = {};
-let babel = babelCore;
+let babel = null;
 let piratesRevert = null;
 
 function installSourceMapSupport() {
@@ -102,7 +102,7 @@ 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;
+  babel = opts.babel ? opts.babel : babelCore;
 
   if (opts.extensions) hookExtensions(opts.extensions);
 

packages/babel-register/test/index.js

@@ -119,4 +119,20 @@ describe("@babel/register", function() {
 
     chai.expect(require(DATA_ES2015)).to.equal("hello");
   });
+
+  it("Resets babel next register", () => {
+    function crash() {
+      throw new Error("shouldn't be called");
+    }
+
+    setupRegister({
+      babel: {
+        transform: crash,
+        getEnv: crash,
+      },
+    });
+    setupRegister();
+
+    chai.expect(require(DATA_ES2015)).to.be.ok;
+  });
 });