Remove Babel (#1074)

* Test helpers no longer rely on Babel.

* Behaviours are no longer imported.

* Removed Babel dependency.

* Fixed linter errors.

test/helpers/assertRevert.js

@@ -1,4 +1,4 @@
-export default async promise => {
+async function assertRevert (promise) {
   try {
     await promise;
     assert.fail('Expected revert not received');
@@ -6,4 +6,8 @@ export default async promise => {
     const revertFound = error.message.search('revert') >= 0;
     assert(revertFound, `Expected "revert", got ${error} instead`);
   }
+}
+
+module.exports = {
+  assertRevert,
 };