setFlag(value) {
    var attr = this.attr;
    attr.setFlag = value;
  }
The "set-flag" property allows a "flag" to be added to a Node instance
at compile time if the attribute is found on the node. This is helpful
if an attribute uses a pattern and a transformer wants to have a simple
check to see if the Node has an attribute that matched the pattern.

Example:

"set-flag": "myCustomFlag"

A Node instance can be checked if it has a flag set as shown below:

if (node.hasFlag('myCustomFlag')) { ... }