feat(aio): cache external resources in the ServiceWorker

Cache resources that are necessary for displaying the basic pages when offline.

aio/ngsw-manifest.json

@@ -1,4 +1,14 @@
 {
+  "external": {
+    "urls": [
+      {"url": "https://fonts.googleapis.com/css?family=Droid+Sans+Mono"},
+      {"url": "https://fonts.gstatic.com/s/droidsansmono/v7/ns-m2xQYezAtqh7ai59hJYdJ2JT0J65PSe7wdxAnx_I.woff2"},
+      {"url": "https://fonts.googleapis.com/icon?family=Material+Icons"},
+      {"url": "https://fonts.gstatic.com/s/materialicons/v22/2fcrYFNaTjcS6g4U3t-Y5ZjZjT5FdEJ140U2DJYC3mY.woff2"},
+      {"url": "https://maxcdn.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.min.css"},
+      {"url": "https://unpkg.com/[email protected]/lunr.min.js"}
+    ]
+  },
   "static.ignore": [
     "\\.js\\.map$",
     "^/assets/images/.*/unused/",

aio/src/app/search/search-worker.js

@@ -5,7 +5,8 @@
 
 var SEARCH_TERMS_URL = '/content/docs/app/search-data.json';
 
-importScripts('https://unpkg.com/[email protected]');
+// NOTE: This needs to be kept in sync with `ngsw-manifest.json`.
+importScripts('https://unpkg.com/[email protected]/lunr.min.js');
 
 var index = createIndex();
 var pages = {};

aio/src/index.html

@@ -20,9 +20,11 @@
   <link rel="icon" type="image/png" href="assets/images/favicons/favicon-16x16.png"
         sizes="16x16">
 
+  <!-- NOTE: These need to be kept in sync with `ngsw-manifest.json`. -->
   <link href="https://fonts.googleapis.com/icon?family=Material+Icons" rel="stylesheet">
   <link href="https://fonts.googleapis.com/css?family=Droid+Sans+Mono" rel="stylesheet">
   <link href="https://maxcdn.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.min.css"rel="stylesheet">
+  <!-- -->
 
   <link rel="manifest" href="pwa-manifest.json">
   <meta name="theme-color" content="#1976d2">