fix #27259, implement covariance checking for strong mode and DDC

pkg/dev_compiler/lib/src/compiler/property_model.dart

 // Copyright (c) 2016, 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 HashMap, HashSet, Queue;
 
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart' show InterfaceType;
 import 'package:analyzer/src/dart/element/element.dart' show FieldElementImpl;
 
@@ skipping 172 matching lines @@
   /// The set of inherited setters, used because JS getters/setters are paired,
   /// so if we're generating a getter we may need to emit a setter that calls
   /// super.
   final inheritedSetters = new HashSet<String>();
 
   final mockMembers = <String, ExecutableElement>{};
 
   final extensionMembers = new Set<ExecutableElement>();
   final mixinExtensionMembers = new Set<ExecutableElement>();
 
-  ClassPropertyModel.build(ExtensionTypeSet extensionTypes,
-      VirtualFieldModel fieldModel, ClassElement classElem) {
+  /// Parameters that are covariant due to covariant generics.
+  final Set<Element> covariantParameters;
+
+  ClassPropertyModel.build(
+      ExtensionTypeSet extensionTypes,
+      VirtualFieldModel fieldModel,
+      ClassElement classElem,
+      this.covariantParameters,
+      Set<ExecutableElement> covariantPrivateMembers) {
     // Visit superclasses to collect information about their fields/accessors.
     // This is expensive so we try to collect everything in one pass.
     for (var base in getSuperclasses(classElem)) {
       for (var accessor in base.accessors) {
         // For getter/setter pairs only process them once.
         if (accessor.correspondingGetter != null) continue;
 
         var field = accessor.variable;
         // Ignore private names from other libraries.
         if (field.isPrivate && accessor.library != classElem.library) {
@@ skipping 21 matching lines @@
     for (var accessor in classElem.accessors) {
       // For getter/setter pairs only process them once.
       if (accessor.correspondingGetter != null) continue;
       // Also ignore abstract fields.
       if (accessor.isAbstract) continue;
 
       var field = accessor.variable;
       var name = field.name;
       // Is it a field?
       if (!field.isSynthetic && field is FieldElementImpl) {
+        var setter = field.setter;
         if (virtualAccessorNames.contains(name) ||
-            fieldModel.isVirtual(field)) {
+            fieldModel.isVirtual(field) ||
+            setter != null &&
+                covariantParameters != null &&
+                covariantParameters.contains(setter.parameters[0]) &&
+                covariantPrivateMembers.contains(setter)) {
           if (field.isStatic) {
             staticFieldOverrides.add(field);
           } else {
             virtualFields[field] = new JS.TemporaryId(name);
           }
         }
       }
     }
   }
 
@@ skipping 73 matching lines @@
       }
     }
 
     visitType(type, false);
 
     for (var m in mockMembers.values) {
       if (possibleExtensions.contains(m.name)) extensionMembers.add(m);
     }
   }
 }