Add --modules=node support

lib/runtime/dart/_js_helper.js

 dart_library.library('dart/_js_helper', null, /* Imports */[
   'dart/_runtime',
   'dart/core',
   'dart/collection',
   'dart/_interceptors',
   'dart/_foreign_helper'
 ], /* Lazy imports */[
 ], function(exports, dart, core, collection, _interceptors, _foreign_helper) {
   'use strict';
   let dartx = dart.dartx;
@@ skipping 1094 matching lines @@
         return !ret.done;
       }
     }
     SyncIterator[dart.implements] = () => [core.Iterator$(E)];
     dart.setSignature(SyncIterator, {
       constructors: () => ({SyncIterator: [SyncIterator$(E), [dart.dynamic]]}),
       methods: () => ({moveNext: [core.bool, []]})
     });
     return SyncIterator;
   });
-  let SyncIterator = SyncIterator$();
+  const SyncIterator = SyncIterator$();
   const _generator = Symbol('_generator');
   const _args = Symbol('_args');
   const SyncIterable$ = dart.generic(function(E) {
     class SyncIterable extends collection.IterableBase$(E) {
       SyncIterable(generator, args) {
         this[_generator] = generator;
         this[_args] = args;
         super.IterableBase();
       }
       [_jsIterator]() {
         return this[_generator](...this[_args]);
       }
       get iterator() {
         return new (SyncIterator$(E))(this[_jsIterator]());
       }
     }
     dart.setSignature(SyncIterable, {
       constructors: () => ({SyncIterable: [SyncIterable$(E), [dart.dynamic, dart.dynamic]]}),
       methods: () => ({[_jsIterator]: [dart.dynamic, []]})
     });
     dart.defineExtensionMembers(SyncIterable, ['iterator']);
     return SyncIterable;
   });
-  let SyncIterable = SyncIterable$();
+  const SyncIterable = SyncIterable$();
   // Exports:
   exports.NoThrows = NoThrows;
   exports.NoInline = NoInline;
   exports.Native = Native;
   exports.JsPeerInterface = JsPeerInterface;
   exports.SupportJsExtensionMethods = SupportJsExtensionMethods;
   exports.defineProperty = defineProperty;
   exports.regExpGetNative = regExpGetNative;
   exports.regExpGetGlobalNative = regExpGetGlobalNative;
   exports.regExpCaptureCount = regExpCaptureCount;
@@ skipping 43 matching lines @@
   exports.CastErrorImplementation = CastErrorImplementation;
   exports.FallThroughErrorImplementation = FallThroughErrorImplementation;
   exports.RuntimeError = RuntimeError;
   exports.random64 = random64;
   exports.jsonEncodeNative = jsonEncodeNative;
   exports.SyncIterator$ = SyncIterator$;
   exports.SyncIterator = SyncIterator;
   exports.SyncIterable$ = SyncIterable$;
   exports.SyncIterable = SyncIterable;
 });