loader: Add dispose method

In a PWA, the loaderServicePromise cannot be reused across page
navigations; style[amp-extension="amp-loader"] is lost. Add a dispose
method to reset the promise when shadowDoc.close() runs.

extensions/amp-loader/0.1/amp-loader.js

@@ -391,13 +391,15 @@ export class LoaderService {
    * @param {number} initDelay
    * @param {number} elementWidth
    * @param {number} elementHeight
+   * @param {function} unsetLoaderPromise
    */
   initializeLoader(
     element,
     loaderRoot,
     initDelay,
     elementWidth,
-    elementHeight
+    elementHeight,
+    unsetLoaderPromise
   ) {
     // Cap the loader delay so that the loader appears immediately, rather than
     // starting part way through the animation.
@@ -410,10 +412,19 @@ export class LoaderService {
     );
     lb.build();
 
+    this.unsetLoaderPromise_ = unsetLoaderPromise;
+
     setImportantStyles(element, {
       '--loader-delay-offset': `${loaderDelay}ms`,
     });
   }
+
+  /**
+   * Unset loaderServicePromise when dispose is called on service
+   */
+  dispose() {
+    this.unsetLoaderPromise_();
+  }
 }
 
 AMP.extension('amp-loader', '0.1', AMP => {

src/loader.js

@@ -36,6 +36,13 @@ function getLoaderServicePromise(ampDoc, element) {
   return loaderServicePromise;
 }
 
+/**
+ * Restore loaderServicePromise to its initial state.
+ */
+function unsetLoaderPromise() {
+  loaderServicePromise = null;
+}
+
 /**
  * Creates a default "loading indicator" element based on the new design.
  *
@@ -68,7 +75,8 @@ export function createLoaderElement(
       loaderRoot,
       initDelay,
       elementWidth,
-      elementHeight
+      elementHeight,
+      unsetLoaderPromise
     );
   });