fixes #27385, implement virtual fields in DDC

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

Index: pkg/dev_compiler/lib/src/compiler/class_property_model.dart
diff --git a/pkg/dev_compiler/lib/src/compiler/class_property_model.dart b/pkg/dev_compiler/lib/src/compiler/class_property_model.dart
new file mode 100644
index 0000000000000000000000000000000000000000..8319c828ffb49350736ae532ed78e5db0e0f86db
--- /dev/null
+++ b/pkg/dev_compiler/lib/src/compiler/class_property_model.dart
@@ -0,0 +1,95 @@
+// 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 HashSet;
+
+import 'package:analyzer/dart/ast/ast.dart' show Identifier;
+import 'package:analyzer/dart/element/element.dart';
+import 'package:analyzer/src/dart/element/element.dart' show FieldElementImpl;
+
+import '../js_ast/js_ast.dart' as JS;
+import 'element_helpers.dart';
+import 'js_names.dart' as JS;
+
+/// Tracks how fields, getters and setters are represented when emitting JS.
+///
+/// Dart classes have implicit features that must be made explicit:
+///
+/// - virtual fields induce a getter and setter pair.
+/// - getters and setters are independent.
+/// - getters and setters can be overridden.
+///
+class ClassPropertyModel {
+  /// Fields that are virtual, that is, they must be generated as a property
+  /// pair in JavaScript.
+  ///
+  /// The value property stores the symbol used for the field's storage slot.
+  final virtualFields = <FieldElement, JS.TemporaryId>{};
+
+  /// Static fields that are overridden, this does not matter for Dart but in
+  /// JS we need to take care initializing these because JS classes inherit
+  /// statics.
+  final staticFieldOverrides = new HashSet<FieldElement>();
+
+  /// The set of inherited getters, used because JS getters/setters are paired,
+  /// so if we're generating a setter we may need to emit a getter that calls
+  /// super.
+  final inheritedGetters = new HashSet<String>();
+
+  /// 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>();
+
+  ClassPropertyModel.build(
+      ClassElement classElem, Iterable<ExecutableElement> extensionMembers) {
+    // 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;
+        var name = field.name;
+        // Ignore private names from other libraries.
+        if (Identifier.isPrivateName(name) &&
+            accessor.library != classElem.library) {
+          continue;
+        }
+
+        if (field.getter?.isAbstract == false) inheritedGetters.add(name);
+        if (field.setter?.isAbstract == false) inheritedSetters.add(name);
+      }
+    }
+
+    var extensionNames =
+        new HashSet<String>.from(extensionMembers.map((e) => e.name));
+
+    // Visit accessors in the current class, and see if they need to be
+    // generated differently based on the inherited fields/accessors.
+    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) {
+        if (inheritedGetters.contains(name) ||
+            inheritedSetters.contains(name) ||
+            extensionNames.contains(name) ||
+            field.isVirtual) {
+          if (field.isStatic) {
+            staticFieldOverrides.add(field);
+          } else {
+            virtualFields[field] = new JS.TemporaryId(name);
+          }
+        }
+      }
+    }
+  }
+}