add TODO about need to review finalize api.

src/elements/element.html

@@ -98,6 +98,12 @@
         this.__finalized = value;
       }
 
+      // TODO(sorvell): need to work on public api surrouding `finalize`.
+      // Due to meta-programming, it's awkward to make a subclass impl of this.
+      // However, a user might want to call `finalize` prior to define to do
+      // this work eagerly. Need to also decide on `finalizeConfig(config)` and
+      // `finalizeTemplate(template)`. Both are public but have simiarly
+      // awkward subclassing characteristics.
       static _finalize() {
         var proto = this.prototype;
         if (!this._finalized) {