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

test/codegen_expected/sunflower/sunflower.js

-dart_library.library('sunflower', null, /* Imports */[
-  'dart_sdk'
-], function load__sunflower(exports, dart_sdk) {
+define(['dart_sdk'], function(dart_sdk) {
   'use strict';
   const core = dart_sdk.core;
   const html = dart_sdk.html;
   const math = dart_sdk.math;
   const dart = dart_sdk.dart;
   const dartx = dart_sdk.dartx;
   const sunflower = Object.create(null);
   const circle = Object.create(null);
   const painter = Object.create(null);
   let StringToElement = () => (StringToElement = dart.constFn(dart.definiteFunctionType(html.Element, [core.String])))();
@@ skipping 103 matching lines @@
     get canvas() {
       return html.CanvasElement.as(painter.querySelector("#canvas"));
     }
   });
   dart.defineLazy(painter, {
     get context() {
       return html.CanvasRenderingContext2D.as(painter.canvas[dartx.getContext]('2d'));
     }
   });
   // Exports:
-  exports.sunflower = sunflower;
-  exports.circle = circle;
-  exports.painter = painter;
+  return {
+    sunflower: sunflower,
+    circle: circle,
+    painter: painter
+  };
 });
 
 //# sourceMappingURL=sunflower.js.map