fixes #27385, implement virtual fields in DDC

pkg/dev_compiler/lib/src/compiler/js_field_storage.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.
-
-import 'dart:collection' show HashSet;
-
-import 'package:analyzer/dart/ast/ast.dart' show Identifier;
-import 'package:analyzer/dart/element/element.dart';
-
-class PropertyOverrideResult {
-  final bool foundGetter;
-  final bool foundSetter;
-
-  PropertyOverrideResult(this.foundGetter, this.foundSetter);
-}
-
-PropertyOverrideResult checkForPropertyOverride(
-    FieldElement field, List<ClassElement> superclasses) {
-  bool foundGetter = false;
-  bool foundSetter = false;
-
-  for (var superclass in superclasses) {
-    var superprop = getProperty(superclass, field.library, field.name);
-    if (superprop == null) continue;
-
-    var getter = superprop.getter;
-    bool hasGetter = getter != null && !getter.isAbstract;
-    if (hasGetter) foundGetter = true;
-
-    var setter = superprop.setter;
-    bool hasSetter = setter != null && !setter.isAbstract;
-    if (hasSetter) foundSetter = true;
-  }
-
-  return new PropertyOverrideResult(foundGetter, foundSetter);
-}
-
-FieldElement getProperty(
-    ClassElement cls, LibraryElement fromLibrary, String name) {
-  // Properties from a different library are not accessible.
-  if (Identifier.isPrivateName(name) && cls.library != fromLibrary) {
-    return null;
-  }
-  for (var accessor in cls.accessors) {
-    var prop = accessor.variable;
-    if (prop.name == name) return prop;
-  }
-  return null;
-}
-
-List<ClassElement> getSuperclasses(ClassElement cls) {
-  var result = <ClassElement>[];
-  var visited = new HashSet<ClassElement>();
-  while (cls != null && visited.add(cls)) {
-    for (var mixinType in cls.mixins.reversed) {
-      var mixin = mixinType.element;
-      if (mixin != null) result.add(mixin);
-    }
-    var supertype = cls.supertype;
-    if (supertype == null) break;
-
-    cls = supertype.element;
-    result.add(cls);
-  }
-  return result;
-}