dynamic imports fix for non-webpack environments. (#3414)

* Added condition to check whether method exists or not

* Conditional loop based on feature detection

* Added code comments as requested by the author

server/build/babel/plugins/handle-import.js

@@ -7,9 +7,16 @@ import Crypto from 'crypto'
 
 const TYPE_IMPORT = 'Import'
 
+/*
+ Added "typeof require.resolveWeak !== 'function'" check instead of
+ "typeof window === 'undefined'" to support dynamic impports in non-webpack environments.
+ "require.resolveWeak" and "require.ensure" are webpack specific methods.
+ They would fail in Node/CommonJS environments.
+*/
+
 const buildImport = (args) => (template(`
   (
-    typeof window === 'undefined' ?
+    typeof require.resolveWeak !== 'function' ?
       new (require('next/dynamic').SameLoopPromise)((resolve, reject) => {
         eval('require.ensure = function (deps, callback) { callback(require) }')
         require.ensure([], (require) => {