Add tests for sandboxing behavior.

packages/babel-helper-transform-fixture-test-runner/src/index.js

@@ -68,9 +68,11 @@ function runModuleInTestContext(id: string, relativeFilename: string) {
 }
 
 /**
- * Run the given snippet of code inside a CommonJS module
+ * Run the given snippet of code inside a CommonJS module.
+ *
+ * Exposed for unit tests, not for use as an API.
  */
-function runCodeInTestContext(code: string, opts: {filename?: string} = {}) {
+export function runCodeInTestContext(code: string, opts: {filename?: string} = {}) {
   const filename = opts.filename || null;
   const dirname = filename ? path.dirname(filename) : null;
   const req = filename ? ((id) => runModuleInTestContext(id, filename)) : null;

packages/babel-helper-transform-fixture-test-runner/test/index.js

@@ -0,0 +1,17 @@
+import assert from "assert";
+import { runCodeInTestContext } from "..";
+
+describe("helper-transform-fixture-test-runner", function() {
+  it("should not execute code in Node's global context", function() {
+    runCodeInTestContext(`
+      global.foo = 4;
+    `);
+    const foo = global.foo;
+    runCodeInTestContext(`
+      var foo = global.foo;
+      delete global.foo;
+      assert.equal(foo, 4);
+    `);
+    assert.equal(foo, undefined);
+  });
+});