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

test/codegen_expected/map_keys.js

-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;
   const dart = dart_sdk.dart;
   const dartx = dart_sdk.dartx;
   const map_keys = Object.create(null);
   let VoidTodynamic = () => (VoidTodynamic = dart.constFn(dart.definiteFunctionType(dart.dynamic, [])))();
   map_keys.main = function() {
     core.print(dart.map({'1': 2, '3': 4, '5': 6}, core.String, core.int));
     core.print(dart.map([1, 2, 3, 4, 5, 6], core.int, core.int));
     core.print(dart.map({'1': 2, [dart.str`${dart.notNull(math.Random.new().nextInt(2)) + 2}`]: 4, '5': 6}, core.String, core.int));
     let x = '3';
     core.print(dart.map(['1', 2, x, 4, '5', 6], core.String, core.int));
     core.print(dart.map(['1', 2, null, 4, '5', 6], core.String, core.int));
   };
   dart.fn(map_keys.main, VoidTodynamic());
   // Exports:
-  exports.map_keys = map_keys;
+  return {
+    map_keys: map_keys
+  };
 });
 
-//# sourceMappingURL=map_keys.js.map
+//# sourceMappingURL=/Users/jmesserly/src/dev_compiler/test/codegen_expected/map_keys.js.map