Add --modules=node support

test/codegen/expect/collection/src/utils.js

 dart_library.library('collection/src/utils', null, /* Imports */[
   'dart/_runtime',
   'dart/core'
 ], /* Lazy imports */[
 ], function(exports, dart, core) {
   'use strict';
   let dartx = dart.dartx;
   const Pair$ = dart.generic(function(E, F) {
     class Pair extends core.Object {
       Pair(first, last) {
         this.first = first;
         this.last = last;
       }
     }
     dart.setSignature(Pair, {
       constructors: () => ({Pair: [Pair$(E, F), [E, F]]})
     });
     return Pair;
   });
-  let Pair = Pair$();
+  const Pair = Pair$();
   // Exports:
   exports.Pair$ = Pair$;
   exports.Pair = Pair;
 });