gistfile1.txt

// This is in PolymerExpressions prototype

    prepareBinding: function(pathString, name, node) {
      var path = Path.get(pathString);

      if (!isLiteralExpression(pathString) && path.valid) {
        if (path.length == 1) {
          return function(model, node, oneTime) {
            if (oneTime)
              return path.getValueFrom(model);

            var scope = findScope(model, path[0]);
            return new PathObserver(scope, path);
          };
        }
        return; // bail out early if pathString is simple path.
      }

      return prepareBinding(pathString, name, node, this);
    },
    
// You can see that it passes itself into prepareBinding function as filterRegistry
    
      function prepareBinding(expressionText, name, node, filterRegistry) {
    var expression;
    try {
      expression = getExpression(expressionText);
      if (expression.scopeIdent &&
          (node.nodeType !== Node.ELEMENT_NODE ||
           node.tagName !== 'TEMPLATE' ||
           (name !== 'bind' && name !== 'repeat'))) {
        throw Error('as and in can only be used within <template bind/repeat>');
      }
    } catch (ex) {