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

test/codegen_expected/expect.js

-dart_library.library('expect', null, /* Imports */[
-  'dart_sdk'
-], function load__expect(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 expect = Object.create(null);
   let dynamicAnddynamicTobool = () => (dynamicAnddynamicTobool = dart.constFn(dart.definiteFunctionType(core.bool, [dart.dynamic, dart.dynamic])))();
   expect.Expect = class Expect extends core.Object {
     static _truncateString(string, start, end, length) {
       if (dart.notNull(end) - dart.notNull(start) > dart.notNull(length)) {
         end = dart.notNull(start) + dart.notNull(length);
@@ skipping 279 matching lines @@
     constructors: () => ({new: dart.definiteFunctionType(expect.TrustTypeAnnotations, [])})
   });
   expect.AssumeDynamic = class AssumeDynamic extends core.Object {
     new() {
     }
   };
   dart.setSignature(expect.AssumeDynamic, {
     constructors: () => ({new: dart.definiteFunctionType(expect.AssumeDynamic, [])})
   });
   // Exports:
-  exports.expect = expect;
+  return {
+    expect: expect
+  };
 });