fixes #52, fields shadowing getters/setters or other fields

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..d2b30b51c14d012662ff9f2725df63b5a4c34c8c
--- /dev/null
+++ b/lib/src/codegen/js_field_storage.dart
@@ -0,0 +1,90 @@
+// 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 propertyOverrides = new HashSet<PropertyInducingElement>();
+  for (var unit in library.partsThenLibrary) {
+    for (var cls in unit.element.types) {
+      var superclasses = getSuperclasses(cls);
+      var visited = new HashSet<PropertyInducingElement>();
+      for (var accessor in cls.accessors) {
+        var prop = accessor.variable;
+        if (visited.add(prop) && !propertyOverrides.contains(prop)) {
+          checkForPropertyOverride(prop, superclasses, propertyOverrides);
+        }
+      }
+    }
+  }
+
+  // Return only the field overrides.
+  return new HashSet<FieldElement>.from(
+      propertyOverrides.where((a) => a is FieldElement));
+}
+
+void checkForPropertyOverride(PropertyInducingElement prop,
+    List<ClassElement> superclasses,
+    HashSet<PropertyInducingElement> propertyOverrides) {
+
+  // Public fields on public types always need storage, because they could be
+  // extended later in another compilation unit.
+  var library = prop.library;
+  var cls = prop.enclosingElement;
+  if (prop.isPublic && cls.isPublic) {
+    propertyOverrides.add(prop);
+    return;
+  }
+
+  bool found = false;
+  for (var superclass in superclasses) {
+    var superprop = getProperty(superclass, library, prop.name);
+    if (superprop != null) {
+      found = true;
+      // Record that the super property is overridden.
+      if (superprop.library == library) propertyOverrides.add(superprop);
+    }
+  }
+
+  // If this we found a super property, then this property overrides it.
+  if (found) propertyOverrides.add(prop);
+}
+
+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;
+}