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 90%
copy from test/codegen/expect/es6_modules.js
copy to test/codegen/expect/node_modules.js
index e96218a8c70d195b21da8a1c4dcbcf1ba571d9bb..5590ecb00c2a695600065ceb407a575942188a27 100644
--- a/test/codegen/expect/es6_modules.js
+++ b/test/codegen/expect/node_modules.js
@@ -1,6 +1,6 @@
-const exports = {};
-import dart from "./dart/_runtime";
-import core from "./dart/core";
+'use strict';
+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 {}
@@ -48,4 +48,3 @@ exports.B$ = B$;
 exports.B = B;
 exports.f = f;
 exports.constant = constant;
-export default exports;