Add --modules=node support

test/codegen/expect/misc.js

 dart_library.library('misc', null, /* Imports */[
   'dart/_runtime',
   'dart/core'
 ], /* Lazy imports */[
 ], function(exports, dart, core) {
   'use strict';
   let dartx = dart.dartx;
   class _Uninitialized extends core.Object {
     _Uninitialized() {
     }
   }
   dart.setSignature(_Uninitialized, {
     constructors: () => ({_Uninitialized: [_Uninitialized, []]})
   });
   const UNINITIALIZED = dart.const(new _Uninitialized());
   const Generic$ = dart.generic(function(T) {
     class Generic extends core.Object {
       get type() {
         return Generic$();
       }
     }
     return Generic;
   });
-  let Generic = Generic$();
+  const Generic = Generic$();
   class Base extends core.Object {
     Base() {
       this.x = 1;
       this.y = 2;
     }
     ['=='](obj) {
       return dart.is(obj, Base) && dart.equals(dart.dload(obj, 'x'), this.x) && dart.equals(dart.dload(obj, 'y'), this.y);
     }
   }
   class Derived extends core.Object {
@@ skipping 23 matching lines @@
   dart.fn(main);
   // Exports:
   exports.UNINITIALIZED = UNINITIALIZED;
   exports.Generic$ = Generic$;
   exports.Generic = Generic;
   exports.Base = Base;
   exports.Derived = Derived;
   exports.expr = expr;
   exports.main = main;
 });