The `devMode` parameter being passed to the `buildScss` function was
not being used. The `buildScss` function was declared _inside_ the
function in which it is invoked, so the `devMode` variable is already
in scope - it doesn't need to be passed in.
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.
