Adding a mechanism to 'upgrade' the Dart type of elements

tools/dom/src/dartium_CustomElementSupport.dart

+// Copyright (c) 2014, 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.
+
+part of dart.dom.html;
+
+/// Dartium ElementUpgrader implementation.
+class _VMElementUpgrader implements ElementUpgrader {
+  final Type _type;
+  final Type _nativeType;
+
+  _VMElementUpgrader(Document document, Type type, String extendsTag) :
+      _type = type,
+      _nativeType = _validateCustomType(type).reflectedType {
+
+    if (extendsTag == null) {
+      if (_nativeType != HtmlElement) {
+        throw new UnsupportedError('Class must provide extendsTag if base '
+              'native class is not HtmlElement');
+      }
+    } else {
+      if (document.createElement(extendsTag).runtimeType != _nativeType) {
+        throw new UnsupportedError(
+            'extendsTag does not match base native class');
+      }
+    }
+  }
+
+  Element upgrade(Element element) {
+    if (element.runtimeType != _nativeType) {
+      throw new UnsupportedError('Element is incorrect type');
+    }
+    return _Utils.changeElementWrapper(element, _type);
+    return null;
+  }
+}
+
+/// Validates that the custom type is properly formed-
+///
+/// * Is a user-defined class.
+/// * Has a created constructor with zero args.
+/// * Derives from an Element subclass.
+///
+/// Then returns the native base class.
+ClassMirror _validateCustomType(Type type) {
+  ClassMirror cls = reflectClass(type);
+  if (_isBuiltinType(cls)) {
+    throw new UnsupportedError('Invalid custom element from '
+        '${(cls.owner as LibraryMirror).uri}.');
+  }
+
+  var className = MirrorSystem.getName(cls.simpleName);
+  var createdConstructor = cls.declarations[new Symbol('$className.created')];
+  if (createdConstructor == null ||
+      createdConstructor is! MethodMirror ||
+      !createdConstructor.isConstructor) {
+    throw new UnsupportedError(
+        'Class is missing constructor $className.created');
+  }
+
+  if (createdConstructor.parameters.length > 0) {
+    throw new UnsupportedError(
+        'Constructor $className.created must take zero arguments');
+  }
+
+  Symbol objectName = reflectClass(Object).qualifiedName;

[Jennifer Messerly, 2014/03/07 00:00:49]

note: my comments below are before I realized this code was just moved around :)

Still, it might be worth simplifying it up a bit. Up to you.

+  bool isRoot(ClassMirror cls) =>
+      cls == null || cls.qualifiedName == objectName;
+  Symbol elementName = reflectClass(HtmlElement).qualifiedName;

[Jennifer Messerly, 2014/03/07 00:00:49]

Hmmm. It might be more reliable to compare reflectedType instances for identity.
I'm not sure if qualifiedName's are guaranteed to be unique.

+  bool isElement(ClassMirror cls) =>
+      cls != null && cls.qualifiedName == elementName;
+  ClassMirror superClass = cls.superclass;
+  ClassMirror nativeClass = _isBuiltinType(superClass) ? superClass : null;
+  while(!isRoot(superClass) && !isElement(superClass)) {

[Jennifer Messerly, 2014/03/07 00:00:49]

for isRoot, you could just walk up superClass == null right?
We used to have issues on dart2js but I think it works correctly everywhere now.
(and this bit of code is only on Dartium anyway)

+    superClass = superClass.superclass;
+    if (nativeClass == null && _isBuiltinType(superClass)) {
+      nativeClass = superClass;
+    }
+  }
+  return nativeClass;
+}
+
+
+bool _isBuiltinType(ClassMirror cls) {
+  // TODO(vsm): Find a less hackish way to do this.

[Jennifer Messerly, 2014/03/07 00:00:49]

ah, I see. I think instances compare correctly in the mirror system. So if you
wanted to check for dart:html you could do this for example:

reflectClass(HtmlElement).owner == cls.owner


Or dart:core:

reflectClass(Object).owner == cls.owner

+  LibraryMirror lib = cls.owner;
+  String libName = lib.uri.toString();
+  return libName.startsWith('dart:');
+}