Create CSS typings before build. Fixes #251. (#350)

* Create CSS typings before build

* Let's try this.

* Adding comment

* Remove hack from travis

config/add-css-types.js

@@ -0,0 +1,74 @@
+const DtsCreator = require('typed-css-modules');
+const chokidar = require('chokidar');
+const util = require('util');
+const sass = require('node-sass');
+
+const sassRender = util.promisify(sass.render);
+
+async function sassToCss(path) {
+  const result = await sassRender({ file: path });
+  return result.css;
+}
+
+/**
+ * @typedef {Object} Opts
+ * @property {boolean} watch Watch for changes
+ */
+/**
+ * Create typing files for CSS & SCSS.
+ *
+ * @param {string[]} rootPaths Paths to search within
+ * @param {Opts} [opts={}] Options.
+ */
+function addCssTypes(rootPaths, opts = {}) {
+  return new Promise((resolve) => {
+    const { watch = false } = opts;
+
+    const paths = [];
+    const preReadyPromises = [];
+    let ready = false;
+
+    for (const rootPath of rootPaths) {
+      // Look for scss & css in each path.
+      paths.push(rootPath + '/**/*.scss');
+      paths.push(rootPath + '/**/*.css');
+    }
+
+    // For simplicity, the watcher is used even if we're not watching.
+    // If we're not watching, we stop the watcher after the initial files are found.
+    const watcher = chokidar.watch(paths, {
+      // Avoid processing already-processed files.
+      ignored: '*.d.*',
+      // Without this, travis and netlify builds never complete. I'm not sure why, but it might be
+      // related to https://github.com/paulmillr/chokidar/pull/758
+      persistent: watch,
+    });
+
+    function change(path) {
+      const promise = (async function() {
+        const creator = new DtsCreator({ camelCase: true });
+        const result = path.endsWith('.scss') ?
+          await creator.create(path, await sassToCss(path)) :
+          await creator.create(path);
+
+        await result.writeFile();
+      })();
+
+      if (!ready) preReadyPromises.push(promise);
+    }
+
+    watcher.on('change', change);
+    watcher.on('add', change);
+
+    // 'ready' is when events have been fired for file discovery.
+    watcher.on('ready', () => {
+      ready = true;
+      // Wait for the current set of processing to finish.
+      Promise.all(preReadyPromises).then(resolve);
+      // And if we're not watching, close the watcher.
+      if (!watch) watcher.close();
+    });
+  })
+}
+
+module.exports = addCssTypes;