Validating that types which require extendsTag actually provide it

tools/dom/src/native_DOMImplementation.dart

 // Copyright (c) 2012, 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 html;
 
 class _ConsoleVariables {
   Map<String, Object> _data = new Map<String, Object>();
 
   /**
@@ skipping 393 matching lines @@
 
   static void register(Document document, String tag, Type type,
       String extendsTagName) {
     // TODO(vsm): Move these checks into native code.
     ClassMirror cls = reflectClass(type);
     if (_isBuiltinType(cls)) {
       throw new UnsupportedError("Invalid custom element from $libName.");
     }
     var className = MirrorSystem.getName(cls.simpleName);
     if (!cls.constructors.containsKey(new Symbol('$className.created'))) {
-      throw new UnsupportedError('Class is missing constructor $className.created');
+      throw new UnsupportedError(
+          'Class is missing constructor $className.created');
     }
+
+    Symbol objectName = reflectClass(Object).qualifiedName;
+    bool isRoot(ClassMirror cls) =>
+        cls == null || cls.qualifiedName == objectName;
+    Symbol elementName = reflectClass(HtmlElement).qualifiedName;
+    bool isElement(ClassMirror cls) =>
+        cls != null && cls.qualifiedName == elementName;
+    ClassMirror superClass = cls.superclass;
+    ClassMirror nativeClass = _isBuiltinType(superClass) ? superClass : null;
+    while(!isRoot(superClass) && !isElement(superClass)) {
+      superClass = superClass.superclass;
+      if (nativeClass == null && _isBuiltinType(superClass)) {
+        nativeClass = superClass;
+      }
+    }
+    if (extendsTagName == null) {
+      if (nativeClass.reflectedType != HtmlElement) {
+        throw new UnsupportedError('Class must provide extendsTag if base '
+            'native class is not HTMLElement');
+      }
+    }
+
     _register(document, tag, type, extendsTagName);
   }
 
   static void _register(Document document, String tag, Type customType,
       String extendsTagName) native "Utils_register";
 
   static Element createElement(Document document, String tagName) native "Utils_createElement";
 
   static void initializeCustomElement(HtmlElement element) native "Utils_initializeCustomElement";
 }
@@ skipping 146 matching lines @@
   _scheduleImmediateHelper._schedule(callback);
 };
 
 get _pureIsolateScheduleImmediateClosure => ((void callback()) =>
   throw new UnimplementedError("scheduleMicrotask in background isolates "
                                "are not supported in the browser"));
 
 void _initializeCustomElement(Element e) {
   _Utils.initializeCustomElement(e);
 }