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

test/codegen_expected/map_keys.js

Index: test/codegen_expected/map_keys.js
diff --git a/test/codegen_expected/map_keys.js b/test/codegen_expected/map_keys.js
index 75c2a0981045a61865725e3e0687c5bea73c1533..4f7dcee3f703a6ca0282e5dbe1bc8778086f81a3 100644
--- a/test/codegen_expected/map_keys.js
+++ b/test/codegen_expected/map_keys.js
@@ -1,6 +1,4 @@
-dart_library.library('map_keys', null, /* Imports */[
-  'dart_sdk'
-], function load__map_keys(exports, dart_sdk) {
+define(['dart_sdk'], function(dart_sdk) {
   'use strict';
   const core = dart_sdk.core;
   const math = dart_sdk.math;
@@ -18,7 +16,9 @@ dart_library.library('map_keys', null, /* Imports */[
   };
   dart.fn(map_keys.main, VoidTodynamic());
   // Exports:
-  exports.map_keys = map_keys;
+  return {
+    map_keys: map_keys
+  };
 });
 
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