Remove virtualField helper, just emit getters/setters in codegen.

test/codegen/expect/fieldtest.js

 dart_library.library('fieldtest', null, /* Imports */[
   'dart_sdk'
 ], function(exports, dart_sdk) {
   'use strict';
   const core = dart_sdk.core;
   const dart = dart_sdk.dart;
   const dartx = dart_sdk.dartx;
   const fieldtest = Object.create(null);
   fieldtest.A = class A extends core.Object {
     A() {
@@ skipping 51 matching lines @@
   fieldtest.BaseWithGetter = class BaseWithGetter extends core.Object {
     get foo() {
       return 1;
     }
   };
   fieldtest.Derived = class Derived extends fieldtest.BaseWithGetter {
     Derived() {
       this.foo = 2;
       this.bar = 3;
     }
+    get foo() {
+      return this[foo];
+    }
+    set foo(value) {
+      this[foo] = value;
+    }
   };
-  dart.virtualField(fieldtest.Derived, 'foo');
+  const foo = Symbol(fieldtest.Derived.name + "." + 'foo'.toString());
   fieldtest.Generic$ = dart.generic(T => {
     class Generic extends core.Object {
       foo(t) {
         dart.as(t, T);
         return core.print(dart.notNull(fieldtest.Generic.bar) + dart.notNull(dart.as(t, core.String)));
       }
     }
     dart.setSignature(Generic, {
       methods: () => ({foo: [dart.dynamic, [T]]})
     });
@@ skipping 50 matching lines @@
     fieldtest.foo(a);
     fieldtest.bar(a);
     core.print(fieldtest.baz(a));
     core.print(new (fieldtest.Generic$(core.String))().foo(' world'));
     core.print(fieldtest.MyEnum.values);
   };
   dart.fn(fieldtest.main, dart.void, []);
   // Exports:
   exports.fieldtest = fieldtest;
 });