fix issue 12190 -- better error message

pkg/mdv/lib/src/element.dart

 // Copyright (c) 2013, 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 mdv;
 
 /** Extensions to the [Element] API. */
 class _ElementExtension extends _NodeExtension {
   _ElementExtension(Element node) : super(node);
 
   // TODO(jmesserly): should path be optional, and default to empty path?
   // It is used that way in at least one path in JS TemplateElement tests
   // (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 {
   final bool conditional;
 
   _AttributeBinding._(node, name, model, path, this.conditional)
       : super(node, name, model, path);
 
   factory _AttributeBinding(Element node, name, model, path) {
     bool conditional = name.endsWith('?');
@@ skipping 13 matching lines @@
       } else {
         node.xtag.attributes.remove(property);
       }
     } else {
       // TODO(jmesserly): escape value if needed to protect against XSS.
       // See https://github.com/polymer-project/mdv/issues/58
       node.xtag.attributes[property] = sanitizeBoundValue(value);
     }
   }
 }