Switch finalized tracking from Set to string property.

Also moves the finalized check earlier, which optimizes out unnecessary
super calls.

src/lib/updating-element.ts

@@ -191,6 +191,13 @@ type UpdateState = typeof STATE_HAS_UPDATED|typeof STATE_UPDATE_REQUESTED|
     typeof STATE_IS_REFLECTING_TO_ATTRIBUTE|
     typeof STATE_IS_REFLECTING_TO_PROPERTY|typeof STATE_HAS_CONNECTED;
 
+/**
+ * The Closure JS Compiler doesn't currently have good support for static
+ * properties semantics where "this" is dynamic, so we use this hack to bypass
+ * any rewriting by the compiler.
+ */
+const finalized = '__finalized__';
+
 /**
  * Base element class which manages element properties and attributes. When
  * properties change, the `update` method is asynchronously called. This method
@@ -210,12 +217,6 @@ export abstract class UpdatingElement extends HTMLElement {
    */
   private static _attributeToPropertyMap: AttributeMap;
 
-  /**
-   * This property was previously used to record the classes that have already
-   * been finalized. This property is no longer used, but is still declared here
-   * for backwards compatability in TypeScript typings.
-   */
-  protected static finalized: boolean|undefined;
   /**
    * Memoized list of all class properties, including any superclass properties.
    * Created lazily on user subclasses when finalizing the class.
@@ -316,15 +317,12 @@ export abstract class UpdatingElement extends HTMLElement {
    * @nocollapse
    */
   protected static finalize() {
-    if (finalized.has(this)) {
-      return;
-    }
     // finalize any superclasses
     const superCtor = Object.getPrototypeOf(this);
-    if (typeof superCtor.finalize === 'function') {
+    if (!superCtor.hasOwnProperty(finalized)) {
       superCtor.finalize();
     }
-    finalized.add(this);
+    (this as unknown as {[key: string]: unknown})[finalized] = true;
     this._ensureClassProperties();
     // initialize Map populated in observedAttributes
     this._attributeToPropertyMap = new Map();
@@ -794,8 +792,5 @@ export abstract class UpdatingElement extends HTMLElement {
   }
 }
 
-/**
- * Marks class as having finished creating properties.
- */
-const finalized = new Set<typeof UpdatingElement>();
-finalized.add(UpdatingElement);
+// UpdatingElement doesn't need finalization.
+(UpdatingElement as unknown as {[key: string]: unknown})[finalized] = true;

src/lit-element.ts

@@ -79,11 +79,9 @@ export class LitElement extends UpdatingElement {
 
   /** @nocollapse */
   protected static finalize() {
-    if (this !== LitElement) {
-      // The Closure JS Compiler does not always preserve the correct "this"
-      // when calling static super methods, so explicitly bind here.
-      super.finalize.apply(this);
-    }
+    // The Closure JS Compiler does not always preserve the correct "this"
+    // when calling static super methods, so explicitly bind here.
+    super.finalize.apply(this);
     // Prepare styling that is stamped at first render time. Styling
     // is built from user provided `styles` or is inherited from the superclass.
     this._styles =
@@ -236,3 +234,6 @@ export class LitElement extends UpdatingElement {
   protected render(): TemplateResult|void {
   }
 }
+
+// LitElement doesn't need finalizing.
+(LitElement as unknown as {[key: string]: unknown})['__finalized__'] = true;