build(aio): implement prerendering

The current implementation is based on @igorminar's [angular-io-v42][1]. It is
using Protractor to request all docs URLs, let them fallback to `/index.html`
and save the rendered page.

[1]: https://github.com/IgorMinar/angular-io-v42/tree/05508ab3/tools/prerenderer

Fixes #15104

aio/tools/prerender/create-specs.js

@@ -0,0 +1,89 @@
+'use strict';
+
+// Imports
+const fs = require('fs');
+const path = require('path');
+const sh = require('shelljs');
+const { BASE_URL, BROWSER_INSTANCES, INPUT_DIR, PORT, TMP_OUTPUT_DIR, TMP_SPECS_DIR } = require('./constants');
+
+sh.config.fatal = true;
+
+// Helpers
+const chunkArray = (items, numChunks) => {
+  numChunks = Math.min(numChunks, items.length);
+  const itemsPerChunk = Math.ceil(items.length / numChunks);
+  const chunks = new Array(numChunks);
+
+  console.log(`Chunking ${items.length} items into ${numChunks} chunks.`);
+
+  for (let i = 0; i < numChunks; i++) {
+    chunks[i] = items.slice(i * itemsPerChunk, (i + 1) * itemsPerChunk);
+  }
+
+  return chunks;
+};
+
+const getAllFiles = rootDir => fs.readdirSync(rootDir).reduce((files, file) => {
+  const absolutePath = path.join(rootDir, file);
+  const isFile = fs.lstatSync(absolutePath).isFile();
+
+  return files.concat(isFile ? absolutePath : getAllFiles(absolutePath));
+}, []);
+
+const getAllUrls = rootDir => getAllFiles(rootDir).
+  filter(absolutePath => path.extname(absolutePath) === '.json').
+  map(absolutePath => absolutePath.slice(0, -5)).
+  map(absolutePath => path.relative(INPUT_DIR, absolutePath)).
+  map(relativePath => `${BASE_URL}/${relativePath}`);
+
+const getTestForChunk = (chunk, idx) => `
+  'use strict';
+
+  const fs = require('fs');
+  const path = require('path');
+  const protractor = require('protractor');
+  const sh = require('shelljs');
+  const url = require('url');
+
+  const browser = protractor.browser;
+  sh.config.fatal = true;
+
+  describe('chunk ${idx}', () => ${JSON.stringify(chunk)}.forEach(urlToPage => {
+    const parsedUrl = url.parse(urlToPage);
+
+    it(\`should render \${parsedUrl.path}\`, done => {
+      browser.get(urlToPage);
+      browser.getPageSource()
+        .then(source => {
+          if (/document not found/i.test(source) && !/file-not-found/i.test(urlToPage)) {
+            return Promise.reject(\`404 for \${urlToPage}\`);
+          }
+
+          const relativeFilePath = parsedUrl.path.replace(/\\/$/, '/index').replace(/^\\//, '') + '.html';
+          const absoluteFilePath = path.resolve('${TMP_OUTPUT_DIR}', relativeFilePath);
+          const absoluteDirPath = path.dirname(absoluteFilePath);
+
+          console.log(\`Writing to \${absoluteFilePath}...\`);
+
+          sh.mkdir('-p', absoluteDirPath);
+          fs.writeFileSync(absoluteFilePath, source);
+        })
+        .then(done, done.fail);
+    });
+  }));
+`;
+
+// Run
+const docsUrls = getAllUrls(INPUT_DIR);
+const chunked = chunkArray(docsUrls, BROWSER_INSTANCES);
+
+sh.rm('-rf', TMP_OUTPUT_DIR);
+sh.rm('-rf', TMP_SPECS_DIR);
+sh.mkdir('-p', TMP_SPECS_DIR);
+
+chunked.forEach((chunk, idx) => {
+  const outputFile = path.join(TMP_SPECS_DIR, `chunk${idx}.spec.js`);
+  const testContent = getTestForChunk(chunk, idx);
+
+  fs.writeFileSync(outputFile, testContent);
+});