fix #27643, failed to virtualize fields if an abstract one is mixed in

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 {
@@ skipping 12 matching lines @@
     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;
-
-    // Stop if this is an abstract getter/setter
-    // TODO(jmesserly): why were we doing this?
-    if (!hasGetter && !hasSetter) break;
   }
 
   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;
@@ skipping 14 matching lines @@
       if (mixin != null) result.add(mixin);
     }
     var supertype = cls.supertype;
     if (supertype == null) break;
 
     cls = supertype.element;
     result.add(cls);
   }
   return result;
 }