Refactor window wrapper for node testing

src/core/window.js

@@ -1,11 +1,16 @@
-var win = {
+function makeWindow() {
+  var win = {
     location: {},
     history: {},
     open: () => {},
     close: () => {}
-}
+  }
 
-try {
+  if(typeof window === "undefined") {
+    return win
+  }
+
+  try {
     win = window
     var props = ["File", "Blob", "FormData"]
     for (var prop of props) {
@@ -13,8 +18,11 @@ try {
         win[prop] = window[prop]
       }
     }
-} catch( e ) {
+  } catch( e ) {
     console.error(e)
+  }
+
+  return win
 }
 
-export default win
+module.exports = makeWindow()