code: function getKey(propNode) {
      if (propNode.key.type === "Literal") {
          return propNode.key.value;
      }
  
      if (propNode.key.type === "Identifier") {
          return propNode.key.name;
      }
  }
docs: Returns a key value of a property expression
  @param {Object} propNode an AST node of a propery expression