Adding validation that Custom element constructor has zero args

tools/dom/src/native_DOMImplementation.dart

Index: tools/dom/src/native_DOMImplementation.dart
diff --git a/tools/dom/src/native_DOMImplementation.dart b/tools/dom/src/native_DOMImplementation.dart
index cbea9eda1b4dc69f198b662d1de38e16057cb770..0cc189c638e1f8749aba307227314124803f02eb 100644
--- a/tools/dom/src/native_DOMImplementation.dart
+++ b/tools/dom/src/native_DOMImplementation.dart
@@ -412,8 +412,14 @@ class _Utils {
       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 $className.created');
+    var createdConstructor = cls.constructors[new Symbol('$className.created')];
+    if (createdConstructor == null) {
+      throw new UnsupportedError(
+          'Class is missing constructor $className.created');
+    }
+    if (createdConstructor.parameters.length > 0) {
+      throw new UnsupportedError(
+          'Constructor $className.created must take zero arguments');
     }
     _register(document, tag, type, extendsTagName);
   }