Add --modules=node support

lib/runtime/dart/_js_helper.js

Index: lib/runtime/dart/_js_helper.js
diff --git a/lib/runtime/dart/_js_helper.js b/lib/runtime/dart/_js_helper.js
index b82c3a7c21008af227978e537ff031bee499e715..147a7cc9da2c1887e3f80882c7b8e1a61faacb8d 100644
--- a/lib/runtime/dart/_js_helper.js
+++ b/lib/runtime/dart/_js_helper.js
@@ -1112,7 +1112,7 @@ dart_library.library('dart/_js_helper', null, /* Imports */[
     });
     return SyncIterator;
   });
-  let SyncIterator = SyncIterator$();
+  const SyncIterator = SyncIterator$();
   const _generator = Symbol('_generator');
   const _args = Symbol('_args');
   const SyncIterable$ = dart.generic(function(E) {
@@ -1136,7 +1136,7 @@ dart_library.library('dart/_js_helper', null, /* Imports */[
     dart.defineExtensionMembers(SyncIterable, ['iterator']);
     return SyncIterable;
   });
-  let SyncIterable = SyncIterable$();
+  const SyncIterable = SyncIterable$();
   // Exports:
   exports.NoThrows = NoThrows;
   exports.NoInline = NoInline;