fixes #69, avoid module name inside module scope

test/codegen/expect/_js_primitives/_js_primitives.js

 var _js_primitives;
-(function(_js_primitives) {
+(function(exports) {
   'use strict';
   // Function printString: (String) → void
   function printString(string) {
     if (typeof dartPrint == "function") {
       dartPrint(string);
       return;
     }
     if (dart.notNull(typeof console == "object") && dart.notNull(typeof console.log != "undefined")) {
       console.log(string);
       return;
     }
     if (typeof window == "object") {
       return;
     }
     if (typeof print == "function") {
       print(string);
       return;
     }
     throw "Unable to print message: " + String(string);
   }
   // Exports:
-  _js_primitives.printString = printString;
+  exports.printString = printString;
 })(_js_primitives || (_js_primitives = {}));