fix issue 12190 -- better error message

pkg/mdv/lib/src/element.dart

Index: pkg/mdv/lib/src/element.dart
diff --git a/pkg/mdv/lib/src/element.dart b/pkg/mdv/lib/src/element.dart
index a126161a71ca51b2ee4da7de005220001292a5cb..d1f62da358c03c0bff0bc6118020b2d625c0522f 100644
--- a/pkg/mdv/lib/src/element.dart
+++ b/pkg/mdv/lib/src/element.dart
@@ -13,6 +13,10 @@ class _ElementExtension extends _NodeExtension {
   // (see "BindImperative" test in original JS code).
   NodeBinding createBinding(String name, model, String path) =>
       new _AttributeBinding(node, name, model, path);
+
+  // Normally we issue this error in Element._ensureTemplate, but we
+  // avoid calling that from the model getter, so issue the error here.
+  get model => throw new UnsupportedError('$node is not a template.');
 }
 
 class _AttributeBinding extends NodeBinding {