Do not allow abstract classes to be registered as custom 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;
@@ skipping 31 matching lines @@
 ///
 /// 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);
+  if (cls.isAbstract) {
+    throw new UnsupportedError('Invalid custom element '
+        'class $className is abstract.');
+  }
+
   var createdConstructor = cls.declarations[new Symbol('$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(
@@ skipping 17 matching lines @@
   return nativeClass;
 }
 
 
 bool _isBuiltinType(ClassMirror cls) {
   // TODO(vsm): Find a less hackish way to do this.
   LibraryMirror lib = cls.owner;
   String libName = lib.uri.toString();
   return libName.startsWith('dart:');
 }