Add --modules=node support

lib/runtime/dart/js.js

Index: lib/runtime/dart/js.js
diff --git a/lib/runtime/dart/js.js b/lib/runtime/dart/js.js
index de81b79fc2b8464b56030b5a7948c34bee836c3f..718050d80fbda455ba7e6ab22f46f5e347da26dd 100644
--- a/lib/runtime/dart/js.js
+++ b/lib/runtime/dart/js.js
@@ -306,7 +306,7 @@ dart_library.library('dart/js', null, /* Imports */[
     ]);
     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;
   }
@@ -406,7 +406,7 @@ dart_library.library('dart/js', null, /* Imports */[
     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;