Add js extention to helper file (#296)

Author: mikeheddes

src/constants.js

@@ -4,4 +4,4 @@ export const EXTERNAL = {};
 
 // NOTE: DO NOT REMOVE the null character `\0` as it may be used by other plugins
 // e.g. https://github.com/rollup/rollup-plugin-node-resolve/blob/313a3e32f432f9eb18cc4c231cc7aac6df317a51/src/index.js#L74
-export const HELPERS = '\0rollupPluginBabelHelpers';
+export const HELPERS = '\0rollupPluginBabelHelpers.js';

test/test.js

@@ -6,7 +6,7 @@ var jsonPlugin = require('rollup-plugin-json');
 var babelPlugin = require('..');
 
 // from ./src/constants
-var HELPERS = '\0rollupPluginBabelHelpers';
+var HELPERS = '\0rollupPluginBabelHelpers.js';
 
 require('source-map-support').install();