fix #626, add AMD module format and make it default

test/codegen_expected/script.js

-dart_library.library('script', null, /* Imports */[
-  'dart_sdk'
-], function load__script(exports, dart_sdk) {
+define(['dart_sdk'], function(dart_sdk) {
   'use strict';
   const core = dart_sdk.core;
   const dart = dart_sdk.dart;
   const dartx = dart_sdk.dartx;
   const script = Object.create(null);
   let ListOfString = () => (ListOfString = dart.constFn(core.List$(core.String)))();
   let ListOfStringTovoid = () => (ListOfStringTovoid = dart.constFn(dart.definiteFunctionType(dart.void, [ListOfString()])))();
   script.main = function(args) {
     let name = args[dartx.join](' ');
     if (name == '') name = 'world';
     core.print(dart.str`hello ${name}`);
   };
   dart.fn(script.main, ListOfStringTovoid());
   // Exports:
-  exports.script = script;
+  return {
+    script: script
+  };
 });