Do not allow abstract classes to be registered as custom elements

tools/dom/src/dartium_CustomElementSupport.dart

Index: tools/dom/src/dartium_CustomElementSupport.dart
diff --git a/tools/dom/src/dartium_CustomElementSupport.dart b/tools/dom/src/dartium_CustomElementSupport.dart
index 3afc56ae4f26580000b18c05fbc3a1430e3f6b8e..a14e07f114f012e3a43b1ff96e4a9d306dec1e8d 100644
--- a/tools/dom/src/dartium_CustomElementSupport.dart
+++ b/tools/dom/src/dartium_CustomElementSupport.dart
@@ -49,6 +49,11 @@ 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 ||