Add --modules=node support

lib/runtime/dart/js.js

 dart_library.library('dart/js', null, /* Imports */[
   'dart/_runtime',
   'dart/core',
   'dart/collection',
   'dart/_js_helper'
 ], /* Lazy imports */[
 ], function(exports, dart, core, collection, _js_helper) {
   'use strict';
   let dartx = dart.dartx;
   const _global = dart.global;
@@ skipping 288 matching lines @@
       'removeAt',
       'removeLast',
       'removeRange',
       'setRange',
       'sort',
       'length',
       'length'
     ]);
     return JsArray;
   });
-  let JsArray = JsArray$();
+  const JsArray = JsArray$();
   function _isBrowserType(o) {
     return o instanceof Blob || o instanceof Event || window.KeyRange && o instanceof KeyRange || o instanceof ImageData || o instanceof Node || window.TypedData && o instanceof TypedData || o instanceof Window;
   }
   dart.fn(_isBrowserType, core.bool, [dart.dynamic]);
   const _dartObj = Symbol('_dartObj');
   class _DartObject extends core.Object {
     _DartObject(dartObj) {
       this[_dartObj] = dartObj;
     }
   }
@@ skipping 79 matching lines @@
       }, core.Function);
       exports._interopCaptureThisExpando.set(f, ret);
     }
     return ret;
   }
   dart.fn(allowInteropCaptureThis, core.Function, [core.Function]);
   const __CastType0$ = dart.generic(function(E) {
     const __CastType0 = dart.typedef('__CastType0', () => dart.functionType(dart.dynamic, [E]));
     return __CastType0;
   });
-  let __CastType0 = __CastType0$();
+  const __CastType0 = __CastType0$();
   // Exports:
   exports.JsObject = JsObject;
   exports.JsFunction = JsFunction;
   exports.JsArray$ = JsArray$;
   exports.JsArray = JsArray;
   exports.allowInterop = allowInterop;
   exports.allowInteropCaptureThis = allowInteropCaptureThis;
 });