This comment was originally written by @zoechi

is this related to https://code.google.com/p/dart/issues/detail?id=20286 ?

This comment was originally written by @jolleekin

I don't think so. Polymer doesn't use PropertyPath.

Thanks for the detailed bug description. This makes sense now.

Re (1): that is also how the JS implementation is written. We have to follow up with them to make sure we are consistent.

Removed Priority-Unassigned label.
Added Priority-Medium, Pkg-Observe, Area-Pkg, PolymerMilestone-Next, Triaged labels.

This comment was originally written by @jolleekin

I opened a new issue for the JS implementation.

googlearchive/observe-js#64

i think this is fixed, https://github.com/dart-lang/observe/blob/master/lib/src/path_observer.dart#L116

if not it's now possible to pass in a list to PropertyPath

['foo', 'bar', '+baz']

Added AssumedStale label.

This comment was originally written by @jolleekin

It's not fixed though. Take a look at [_setObjectProperty]. This method doesn't check for strings. If you pass a list that contains strings and Symbols, [_setObjectProperty] will fail.

  bool _setObjectProperty(object, property, value) {
    if (object == null) return false;

    if (property is int) {
      if (object is List && property >= 0 && property < object.length) {
        object[property] = value;
        return true;
      }
    } else if (property is Symbol) {
      // Support indexer if available, e.g. Maps or polymer_expressions Scope.
      if (object is Indexable || object is Map && !_MAP_PROPERTIES.contains(property)) {
        object[smoke.symbolToName(property)] = value;
        return true;
      }
      try {
        smoke.write(object, property, value);
        return true;
      } on NoSuchMethodError catch (e, s) {
        if (!smoke.hasNoSuchMethod(object.runtimeType)) rethrow;
      }
    }

    if (_logger.isLoggable(Level.FINER)) {
      _logger.finer("can't set $property in $object");
    }
    return false;
  }

To fix the problem, [_setObjectProperty] needs to handle strings as follows.

  bool _setObjectProperty(object, property, value) {
    if (object == null) return false;

    if (property is int) {
      if (object is List && property >= 0 && property < object.length) {
        object[property] = value;
        return true;
      }
    } else if (property is String) {
      if (object is Indexable || object is Map && !_MAP_PROPERTIES.contains(property)) {
        object[property] = value;
        return true;
      }
    } else if (property is Symbol) {
      try {
        smoke.write(object, property, value);
        return true;
      } on NoSuchMethodError catch (e, s) {
        if (!smoke.hasNoSuchMethod(object.runtimeType)) rethrow;
      }
    }

    if (_logger.isLoggable(Level.FINER)) {
      _logger.finer("can't set $property in $object");
    }
    return false;
  }

This issue has been moved to dart-archive/observe#68.