Add --modules=node support

test/codegen/expect/collection/priority_queue.js

Index: test/codegen/expect/collection/priority_queue.js
diff --git a/test/codegen/expect/collection/priority_queue.js b/test/codegen/expect/collection/priority_queue.js
index 106eb53caf98ae299ef5a03760befa4d038d2502..a17ee31deaf524c3aee8f30714ee721b5bdd5405 100644
--- a/test/codegen/expect/collection/priority_queue.js
+++ b/test/codegen/expect/collection/priority_queue.js
@@ -10,7 +10,7 @@ dart_library.library('collection/priority_queue', null, /* Imports */[
     class PriorityQueue extends core.Object {}
     return PriorityQueue;
   });
-  let PriorityQueue = PriorityQueue$();
+  const PriorityQueue = PriorityQueue$();
   const _queue = Symbol('_queue');
   const _length = Symbol('_length');
   const _add = Symbol('_add');
@@ -226,13 +226,13 @@ dart_library.library('collection/priority_queue', null, /* Imports */[
     });
     return HeapPriorityQueue;
   });
-  let HeapPriorityQueue = HeapPriorityQueue$();
+  const HeapPriorityQueue = HeapPriorityQueue$();
   HeapPriorityQueue._INITIAL_CAPACITY = 7;
   const __CastType0$ = dart.generic(function(E) {
     const __CastType0 = dart.typedef('__CastType0', () => dart.functionType(core.int, [E, E]));
     return __CastType0;
   });
-  let __CastType0 = __CastType0$();
+  const __CastType0 = __CastType0$();
   // Exports:
   exports.PriorityQueue$ = PriorityQueue$;
   exports.PriorityQueue = PriorityQueue;