fix fields that override getters/setters

lib/src/codegen/js_field_storage.dart

Index: lib/src/codegen/js_field_storage.dart
diff --git a/lib/src/codegen/js_field_storage.dart b/lib/src/codegen/js_field_storage.dart
new file mode 100644
index 0000000000000000000000000000000000000000..3a0833e413f505eca3643c13ddb8fdcfae79d0fc
--- /dev/null
+++ b/lib/src/codegen/js_field_storage.dart
@@ -0,0 +1,87 @@
+// 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.
+
+library dev_compiler.src.codegen.js_field_storage;
+
+import 'dart:collection' show HashMap, HashSet;
+import 'package:analyzer/src/generated/ast.dart' show Identifier;
+import 'package:analyzer/src/generated/element.dart';
+
+import 'package:dev_compiler/src/info.dart' show LibraryUnit;
+
+/// We use a storage slot for fields that override or can be overridden by
+/// getter/setter pairs.
+HashSet<FieldElement> findFieldsNeedingStorage(LibraryUnit library) {
+  var overrides = new HashSet<FieldElement>();
+  for (var unit in library.partsThenLibrary) {
+    for (var cls in unit.element.types) {
+      var superclasses = getSuperclasses(cls);
+      for (var field in cls.fields) {
+        if (!field.isSynthetic && !overrides.contains(field)) {
+          checkForPropertyOverride(field, superclasses, overrides);
+        }
+      }
+    }
+  }
+
+  return overrides;
+}
+
+void checkForPropertyOverride(FieldElement field,
+    List<ClassElement> superclasses, HashSet<FieldElement> overrides) {
+  assert(!field.isSynthetic);
+
+  var library = field.library;
+
+  bool found = false;
+  for (var superclass in superclasses) {
+    var superprop = getProperty(superclass, library, field.name);
+    if (superprop != null) {
+      // If we find an abstract getter/setter pair, stop the search.
+      var getter = superprop.getter;
+      var setter = superprop.setter;
+      if ((getter == null || getter.isAbstract) &&
+          (setter == null || setter.isAbstract)) {
+        break;
+      }
+
+      found = true;
+      // Record that the super property is overridden.
+      if (superprop.library == library) overrides.add(superprop);
+    }
+  }
+
+  // If this we found a super property, then this property overrides it.
+  if (found) overrides.add(field);
+}
+
+PropertyInducingElement 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;
+}