Add --modules=node support

test/codegen/expect/node_modules.js

Index: test/codegen/expect/node_modules.js
diff --git a/test/codegen/expect/es6_modules.js b/test/codegen/expect/node_modules.js
similarity index 87%
copy from test/codegen/expect/es6_modules.js
copy to test/codegen/expect/node_modules.js
index 58de87d87f1e0fc5392e45cc46117d65ef50571d..8992f90d28a5b5bcd7a3fc6abbd7fff29043e895 100644
--- a/test/codegen/expect/es6_modules.js
+++ b/test/codegen/expect/node_modules.js
@@ -1,7 +1,6 @@
 'use strict';
-const exports = {};
-import dart from "dart/_runtime";
-import core from "dart/core";
+let dart = require("dart/_runtime");
+let core = require("dart/core");
 let dartx = dart.dartx;
 const Callback = dart.typedef('Callback', () => dart.functionType(dart.void, [], {i: core.int}));
 class A extends core.Object {}
@@ -10,12 +9,12 @@ const B$ = dart.generic(function(T) {
   class B extends core.Object {}
   return B;
 });
-let B = B$();
+const B = B$();
 const _B$ = dart.generic(function(T) {
   class _B extends core.Object {}
   return _B;
 });
-let _B = _B$();
+const _B = _B$();
 function f() {
 }
 dart.fn(f);
@@ -47,4 +46,3 @@ exports.B$ = B$;
 exports.B = B;
 exports.f = f;
 exports.constant = constant;
-export default exports;