Add --modules=node support

test/codegen/expect/fieldtest.js

 dart_library.library('fieldtest', null, /* Imports */[
   'dart/_runtime',
   'dart/core'
 ], /* Lazy imports */[
 ], function(exports, dart, core) {
   'use strict';
   let dartx = dart.dartx;
   class A extends core.Object {
     A() {
       this.x = 42;
     }
   }
   const B$ = dart.generic(function(T) {
     class B extends core.Object {
       B() {
         this.x = null;
         this.y = null;
         this.z = null;
       }
     }
     return B;
   });
-  let B = B$();
+  const B = B$();
   function foo(a) {
     core.print(a.x);
     return a.x;
   }
   dart.fn(foo, core.int, [A]);
   function bar(a) {
     core.print(dart.dload(a, 'x'));
     return dart.as(dart.dload(a, 'x'), core.int);
   }
   dart.fn(bar, core.int, [dart.dynamic]);
@@ skipping 39 matching lines @@
       foo(t) {
         dart.as(t, T);
         return core.print(dart.notNull(Generic$().bar) + dart.notNull(dart.as(t, core.String)));
       }
     }
     dart.setSignature(Generic, {
       methods: () => ({foo: [dart.dynamic, [T]]})
     });
     return Generic;
   });
-  let Generic = Generic$();
+  const Generic = Generic$();
   Generic.bar = 'hello';
   class StaticFieldOrder1 extends core.Object {}
   StaticFieldOrder1.d = 4;
   StaticFieldOrder1.c = dart.notNull(StaticFieldOrder1.d) + 2;
   StaticFieldOrder1.b = dart.notNull(StaticFieldOrder1.c) + 3;
   StaticFieldOrder1.a = dart.notNull(StaticFieldOrder1.b) + 1;
   class StaticFieldOrder2 extends core.Object {}
   StaticFieldOrder2.d = 4;
   StaticFieldOrder2.c = dart.notNull(StaticFieldOrder2.d) + 2;
   StaticFieldOrder2.b = dart.notNull(StaticFieldOrder2.c) + 3;
@@ skipping 35 matching lines @@
   exports.compute = compute;
   exports.BaseWithGetter = BaseWithGetter;
   exports.Derived = Derived;
   exports.Generic$ = Generic$;
   exports.Generic = Generic;
   exports.StaticFieldOrder1 = StaticFieldOrder1;
   exports.StaticFieldOrder2 = StaticFieldOrder2;
   exports.MyEnum = MyEnum;
   exports.main = main;
 });