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

tool/input_sdk/private/ddc_runtime/classes.dart

Index: tool/input_sdk/private/ddc_runtime/classes.dart
diff --git a/tool/input_sdk/private/ddc_runtime/classes.dart b/tool/input_sdk/private/ddc_runtime/classes.dart
index 683aed56ea36990f5418c19bf7777b1c0d4b9fed..adaf1d984f98c1c594bceb7043266b41b401e324 100644
--- a/tool/input_sdk/private/ddc_runtime/classes.dart
+++ b/tool/input_sdk/private/ddc_runtime/classes.dart
@@ -230,27 +230,6 @@ setSignature(f, signature) => JS('', '''(() => {
 hasMethod(obj, name) => JS('', '$getMethodType($obj, $name) !== void 0');
 
 ///
-/// This is called whenever a derived class needs to introduce a new field,
-/// shadowing a field or getter/setter pair on its parent.
-///
-/// This is important because otherwise, trying to read or write the field
-/// would end up calling the getter or setter, and one of those might not even
-/// exist, resulting in a runtime error. Even if they did exist, that's the
-/// wrong behavior if a new field was declared.
-///
-virtualField(subclass, fieldName) => JS('', '''(() => {
-  // If the field is already overridden, do nothing.
-  let prop = $getOwnPropertyDescriptor($subclass.prototype, $fieldName);
-  if (prop) return;
-
-  let symbol = Symbol($subclass.name + '.' + $fieldName.toString());
-  $defineProperty($subclass.prototype, $fieldName, {
-    get: function() { return this[symbol]; },
-    set: function(x) { this[symbol] = x; }
-  });
-})()''');
-
-///
 /// Given a class and an initializer method name, creates a constructor
 /// function with the same name. For example `new SomeClass.name(args)`.
 ///