Fix `yarn build styles:dev` (#9709)

The `styles:dev` step of the development build was broken in #9568 when
the `lockdown()` function from SES was introduced. One of the effects
of `lockdown()` is preventing the use of `RegExp.lastMatch`, which is
relied upon by `postcss` for parsing inline sourcemaps.

This problem has been worked around by re-arranging the build pipeline
for styles.

Firstly, the `autoprefixer` stage was moved _before_ sourcemaps were
generated. Its placement _after_ sourcemaps was a mistake in the first
place - it should have always been before they were written, so that
the sourcemaps could keep track of the changes made by `autoprefixer`.

Second, the `-rtl` sourcemap generation was moved to a separate build
pipeline. The `rtl` stage also uses `postcss`, and it must come after
the sourcemaps are written for the non-RTL styles, so splitting it into
a separate build was the only way to avoid passing in inline
sourcemaps. This does make the build slightly slower, but it also
decreases the size of the RTL sourcemaps dramatically.

development/build/styles.js

@@ -54,22 +54,25 @@ function createStyleTasks ({ livereload }) {
     }
 
     async function buildScss () {
-      await pump(...[
-        // pre-process
-        gulp.src(src),
-        devMode && sourcemaps.init(),
-        sass().on('error', sass.logError),
-        devMode && sourcemaps.write(),
-        autoprefixer(),
-        // standard
-        gulp.dest(dest),
-        // right-to-left
-        rtlcss(),
-        rename({ suffix: '-rtl' }),
-        devMode && sourcemaps.write(),
-        gulp.dest(dest),
-      ].filter(Boolean))
+      await Promise.all([
+        buildScssPipeline(src, dest, devMode, false),
+        buildScssPipeline(src, dest, devMode, true),
+      ])
     }
   }
 
 }
+
+async function buildScssPipeline (src, dest, devMode, rtl) {
+  await pump(...[
+    // pre-process
+    gulp.src(src),
+    devMode && sourcemaps.init(),
+    sass().on('error', sass.logError),
+    autoprefixer(),
+    rtl && rtlcss(),
+    rtl && rename({ suffix: '-rtl' }),
+    devMode && sourcemaps.write(),
+    gulp.dest(dest),
+  ].filter(Boolean))
+}