Do not allow abstract classes to be registered as custom elements

sdk/lib/html/dartium/html_dartium.dart

Index: sdk/lib/html/dartium/html_dartium.dart
diff --git a/sdk/lib/html/dartium/html_dartium.dart b/sdk/lib/html/dartium/html_dartium.dart
index 63d97d759439caaf60b4ab5ea118f0a98b1532a4..bf43c15d97f9b22796609ac317dec4c268a9b7f5 100644
--- a/sdk/lib/html/dartium/html_dartium.dart
+++ b/sdk/lib/html/dartium/html_dartium.dart
@@ -40487,6 +40487,11 @@ ClassMirror _validateCustomType(Type type) {
   }
 
   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 ||