Use no-cors mode for all service worker resources

Some of the resources we load from other origins must be loaded as
opaque resources that can't be read by the application.  In fact, it
turns out that there are no app resources that need to be read by the
app.  So we should load everything in no-cors mode as opaque
resources.  This fixes failures loading and caching MDL resources.

Change-Id: I25ec4228285e3149746d906e6461ac04aeedc7e1

demo/service_worker.js

@@ -143,19 +143,24 @@ const CACHEABLE_URL_PREFIXES = [
 function onInstall(event) {
   const preCacheApplication = async () => {
     const cache = await caches.open(CACHE_NAME);
+    // Fetching these with addAll fails for CORS-restricted content, so we use
+    // fetchAndCache with no-cors mode to work around it.
 
     // Optional resources: failure on these will NOT fail the Promise chain.
     // We will also not wait for them to be installed.
-    // Fetching these with addAll fails for CORS-restricted content, so we use
-    // fetchAndCache with no-cors mode to work around it.
     for (const url of OPTIONAL_RESOURCES) {
       const request = new Request(url, {mode: 'no-cors'});
       fetchAndCache(cache, request).catch(() => {});
     }
 
     // Critical resources: failure on these will fail the Promise chain.
     // The installation will not be complete until these are all cached.
-    return cache.addAll(CRITICAL_RESOURCES);
+    const criticalFetches = [];
+    for (const url of CRITICAL_RESOURCES) {
+      const request = new Request(url, {mode: 'no-cors'});
+      criticalFetches.push(fetchAndCache(cache, request));
+    }
+    return Promise.all(criticalFetches);
   };
 
   event.waitUntil(preCacheApplication());