Add --modules=node support

test/codegen/expect/fieldtest.js

Index: test/codegen/expect/fieldtest.js
diff --git a/test/codegen/expect/fieldtest.js b/test/codegen/expect/fieldtest.js
index c8b14863cee4391f70e38a26c0194b206eda56c7..4d0a6cdabe7983d0db8bfa8ea28b99deb1776fa4 100644
--- a/test/codegen/expect/fieldtest.js
+++ b/test/codegen/expect/fieldtest.js
@@ -20,7 +20,7 @@ dart_library.library('fieldtest', null, /* Imports */[
     }
     return B;
   });
-  let B = B$();
+  const B = B$();
   function foo(a) {
     core.print(a.x);
     return a.x;
@@ -80,7 +80,7 @@ dart_library.library('fieldtest', null, /* Imports */[
     });
     return Generic;
   });
-  let Generic = Generic$();
+  const Generic = Generic$();
   Generic.bar = 'hello';
   class StaticFieldOrder1 extends core.Object {}
   StaticFieldOrder1.d = 4;