Ensure instanceof works after babel ES5 compilation

lib/legacy/legacy-data-mixin.html

@@ -19,6 +19,9 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
     constructor(message) {
       super(message);
       this.name = this.constructor.name;
+      // Affordances for ensuring instanceof works after babel ES5 compilation
+      this.constructor = UndefinedArgumentError;
+      this.__proto__ = UndefinedArgumentError.prototype;
     }
   };