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

test/codegen_expected/varargs.js

-dart_library.library('varargs', null, /* Imports */[
-  'dart_sdk'
-], function load__varargs(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 varargs = Object.create(null);
   const src__varargs = Object.create(null);
   let dynamicAnddynamicTodynamic = () => (dynamicAnddynamicTodynamic = dart.constFn(dart.definiteFunctionType(dart.dynamic, [dart.dynamic, dart.dynamic])))();
   let dynamicTodynamic = () => (dynamicTodynamic = dart.constFn(dart.definiteFunctionType(dart.dynamic, [dart.dynamic])))();
   varargs.varargsTest = function(x, ...others) {
     let args = [1, others];
@@ skipping 11 matching lines @@
   };
   dart.setSignature(src__varargs._Rest, {
     constructors: () => ({new: dart.definiteFunctionType(src__varargs._Rest, [])})
   });
   src__varargs.rest = dart.const(new src__varargs._Rest());
   src__varargs.spread = function(args) {
     dart.throw(new core.StateError('The spread function cannot be called, ' + 'it should be compiled away.'));
   };
   dart.fn(src__varargs.spread, dynamicTodynamic());
   // Exports:
-  exports.varargs = varargs;
-  exports.src__varargs = src__varargs;
+  return {
+    varargs: varargs,
+    src__varargs: src__varargs
+  };
 });