build: patch rules_sass to properly report errors in worker mode

WORKSPACE

@@ -15,6 +15,10 @@ http_archive(
 # Add sass rules
 http_archive(
     name = "io_bazel_rules_sass",
+    # Patch `rules_sass` to work around a bug that causes error messages to be not
+    # printed in worker mode: https://github.com/bazelbuild/rules_sass/issues/96.
+    # TODO(devversion): remove this patch once the Sass Node entry-point returns a `Promise`.
+    patches = ["//tools/bazel:sass_worker_async.patch"],
     sha256 = "c78be58f5e0a29a04686b628cf54faaee0094322ae0ac99da5a8a8afca59a647",
     strip_prefix = "rules_sass-1.25.0",
     urls = [

tools/bazel/sass_worker_async.patch

@@ -0,0 +1,15 @@
+diff --git sass/sass_wrapper.js sass/sass_wrapper.js
+index 21abb8f..168ee49 100644
+--- sass/sass_wrapper.js
++++ sass/sass_wrapper.js
+@@ -17,7 +17,9 @@ const fs = require('fs');
+ const args = process.argv.slice(2);
+ if (runAsWorker(args)) {
+   debug('Starting Sass compiler persistent worker...');
+-  runWorkerLoop(args => sass.run_(args));
++  runWorkerLoop(args => {
++    return new Promise((resolve, reject) => sass.run_(args)['then$1$2$onError'](resolve, reject));
++  });
+   // Note: intentionally don't process.exit() here, because runWorkerLoop
+   // is waiting for async callbacks from node.
+ } else {