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

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 2b281cb32900d780d4d3035bed59c66e16a53927..ee1281a0f62877ec5df8b35e2878dc44d2e5df61 100644
--- a/test/codegen/expect/fieldtest.js
+++ b/test/codegen/expect/fieldtest.js
@@ -69,8 +69,14 @@ dart_library.library('fieldtest', null, /* Imports */[
       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) {