Make it possible for sky-element to register any tag name

sky/engine/bindings/custom/dart_element_custom.cc

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "sky/engine/config.h"
+#include "sky/engine/core/dom/Element.h"
+
+#include "dart/runtime/include/dart_api.h"
+#include "sky/engine/core/script/dom_dart_state.h"
+#include "sky/engine/tonic/dart_converter.h"
+
+namespace blink {
+namespace DartElementInternal {
+
+void constructorCallback(Dart_NativeArguments args) {
+  Dart_Handle receiver = Dart_GetNativeArgument(args, 0);
+  DCHECK(!LogIfError(receiver));
+
+  Dart_Handle tag_name = Dart_GetField(receiver,
+                                       Dart_NewStringFromCString("tagName"));
+  if (!Dart_IsString(tag_name)) {
+    Dart_ThrowException(Dart_NewStringFromCString("tagName is not a string"));
+    return;
+  }
+
+  RefPtr<Element> element = Element::create(
+      QualifiedName(StringFromDart(tag_name)), DOMDartState::CurrentDocument());
+
+  // TODO(abarth): We should remove these states because elements are never
+  // waiting for upgrades.
+  element->setCustomElementState(Element::WaitingForUpgrade);
+  element->setCustomElementState(Element::Upgraded);
+  element->AssociateWithDartWrapper(args);
+}
+
+}  // namespace DartElementInternal
+}  // namespace blink