Fix babel-polyfill related issue in babel example

examples/babel/handler.js

@@ -1,3 +1,7 @@
+if (!global._babelPolyfill) {
+   require('babel-polyfill');
+}
+
 export const hello = (event, context, cb) => {
   const p = new Promise((resolve, reject) => {
     resolve('success');

examples/babel/webpack.config.js

@@ -1,8 +1,5 @@
 module.exports = {
-  entry: [
+  entry: './handler.js',
-    'babel-polyfill',
-    './handler.js'
-  ],
   target: 'node',
   module: {
     loaders: [{