Add --modules=node support

test/codegen/expect/closure.js

 dart_library.library('closure', null, /* Imports */[
   'dart/_runtime',
   'dart/core',
   'dart/js'
 ], /* Lazy imports */[
 ], function(exports, dart, core, js) {
   'use strict';
   let dartx = dart.dartx;
   /** @typedef {function({i: (?number|undefined)}=)} */
   const Callback = dart.typedef('Callback', () => dart.functionType(dart.void, [], {i: core.int}));
@@ skipping 91 matching lines @@
         typed_method: [core.String, [Foo$(), core.List, core.int, core.num, core.double, core.bool, core.String, js.JsArray, js.JsObject, js.JsFunction]],
         optional_params: [dart.dynamic, [dart.dynamic], [dart.dynamic, dart.dynamic]],
         nullary_method: [dart.dynamic, []],
         function_params: [dart.dynamic, [dart.functionType(core.int, [dart.dynamic], [dart.dynamic]), dart.functionType(dart.dynamic, [dart.dynamic], {y: core.String, z: dart.dynamic}), Callback]]
       }),
       statics: () => ({named_params: [dart.dynamic, [dart.dynamic], {b: dart.dynamic, c: dart.dynamic}]}),
       names: ['named_params']
     });
     return Foo;
   });
-  let Foo = Foo$();
+  const Foo = Foo$();
   /** @final {string} */
   Foo.some_static_constant = "abc";
   /** @final {string} */
   Foo.some_static_final = "abc";
   /** @type {string} */
   Foo.some_static_var = "abc";
   class Bar extends core.Object {}
   class Baz extends dart.mixin(Foo$(core.int), Bar) {
     /** @param {?number} i */
     Baz(i) {
@@ skipping 15 matching lines @@
   exports.some_top_level_var = "abc";
   // Exports:
   exports.Callback = Callback;
   exports.Foo$ = Foo$;
   exports.Foo = Foo;
   exports.Bar = Bar;
   exports.Baz = Baz;
   exports.main = main;
   exports.some_top_level_constant = some_top_level_constant;
 });