Add the core amp-story component (#11511)

* Add the core amp-story component

* Add TODO LDAP

* Update amp-story.css

* Update amp-story.js

* Fix eslint errors in amp-story.js and add back experiment

* Fix lint errors in test-amp-story.js

* amp-story.js lint changes

* Update test-amp-story.js

* Update amp-story.js

* Update amp-story.js

* Add @const where possible.

* Add fixes for various integration points that broke during merges.

* Remove renamed 'AnalyticsTrigger'

* Add missing trailing comma to test file

* Update closure type annotations.

* Remove double nullability indicators

* Explicitly import animation typedefs.

* Update typedefs for animation-types

* Partial fixes for closure type issues

* Partial fixes for closure type issues (part 2)

* Partial fixes for closure type issues (part 3)

* Remove extra whitespace

* Partial fixes for closure type issues (part 4)

* Fix missing imports and typos

* Fix missing imports and typos

* Fix remaining type issues

* Skip failing amp-story tests; fix loading screen when no elements require loading.

extensions/amp-story/0.1/amp-story-page.js

@@ -97,10 +97,10 @@ export class AmpStoryPage extends AMP.BaseElement {
   constructor(element) {
     super(element);
 
-    /** @private @const {?AnimationManager} */
+    /** @private {?AnimationManager} */
     this.animationManager_ = null;
 
-    /** @private @const {!Array<!PageElement>} */
+    /** @private {!Array<!PageElement>} */
     this.pageElements_ = [];
 
     /** @private {?function()} */
@@ -215,6 +215,7 @@ export class AmpStoryPage extends AMP.BaseElement {
   markPageAsLoaded_() {
     this.isLoaded_ = true;
     this.element.classList.add(PAGE_LOADED_CLASS_NAME);
+    this.markPageAsShown_();
     this.resolveLoadPromise_();
   }
 
@@ -248,7 +249,7 @@ export class AmpStoryPage extends AMP.BaseElement {
    * @public
    */
   calculateLoadStatus() {
-    if (this.isLoaded_) {
+    if (this.isLoaded_ || this.pageElements_.length == 0) {
       return true;
     }
 
@@ -451,21 +452,22 @@ export class AmpStoryPage extends AMP.BaseElement {
   /**
    * Determines whether the specified element is a valid media element for auto-
    * advance.
-   * @param {?Element} el
+   * @param {?Element} elOrNull
    * @param {!function()} callback
    * @private
    */
-  onMediaElementComplete_(el, callback) {
-    user().assertElement(el, 'ID specified for automatic advance ' +
-        `does not refer to any element on page '${this.element.id}'.`);
+  onMediaElementComplete_(elOrNull, callback) {
+    const el = user().assertElement(elOrNull,
+        'ID specified for automatic advance does not refer to any element' +
+        `on page '${this.element.id}'.`);
 
     const mediaElement = this.getMediaElement_(el);
     if (mediaElement) {
       this.autoAdvanceUnlistenDef_ =
           listenOnce(mediaElement, 'ended', callback);
     } else if (this.isVideoInterfaceVideo_(el)) {
       this.autoAdvanceUnlistenDef_ =
-          listenOnce(el, VideoEvents.ENDED, callback, /* opt_capture */ true);
+          listenOnce(el, VideoEvents.ENDED, callback, {capture: true});
     } else {
       user().error(TAG, `Element with ID ${el.id} is not a media element ` +
           'supported for automatic advancement.');
@@ -602,8 +604,8 @@ export class AmpStoryPage extends AMP.BaseElement {
 
   /**
    * Navigates to the next page in the story.
-   * @param {*} opt_isAutomaticAdvance Whether this navigation was caused by an
-   *     automatic advancement after a timeout.
+   * @param {boolean} opt_isAutomaticAdvance Whether this navigation was caused
+   *     by an automatic advancement after a timeout.
    */
   next(opt_isAutomaticAdvance) {
     this.switchTo_(