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

tool/input_sdk/private/ddc_runtime/classes.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /// This library defines the operations that define and manipulate Dart
 /// classes.  Included in this are:
 ///   - Generics
 ///   - Class metadata
 ///   - Extension methods
 ///
@@ skipping 212 matching lines @@
   $_setConstructorSignature($f, constructors);
   $_setMethodSignature($f, methods);
   $_setStaticSignature($f, statics);
   $_setStaticTypes($f, names);
   $tagMemoized($f, () => $Type);
 })()''');
 
 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)`.
 ///
 defineNamedConstructor(clazz, name) => JS('', '''(() => {
   let proto = $clazz.prototype;
   let initMethod = proto[$name];
   let ctor = function() { return initMethod.apply(this, arguments); };
   ctor.prototype = proto;
   // Use defineProperty so we don't hit a property defined on Function,
   // like `caller` and `arguments`.
@@ skipping 108 matching lines @@
 })()''');
 
 /// Sets the element type of a list literal.
 list(obj, elementType) =>
     JS('', '$setType($obj, ${getGenericClass(JSArray)}($elementType))');
 
 setBaseClass(derived, base) => JS('', '''(() => {
   // Link the extension to the type it's extending as a base class.
   $derived.prototype.__proto__ = $base.prototype;
 })()''');