Can not enable scss build at angular 6.0.1

**I'm submitting a ...** (check one with "x")
```
[ x ] bug report => search github for a similar issue or PR before submitting
[ ] feature request
[ ] support request => Please do not submit support request here, instead see use [gitter](https://gitter.im/mgechev/angular2-seed) or [stackoverflow](https://stackoverflow.com/questions/tagged/angular2)
```

**Current behavior**
I want enable scss
Reference https://github.com/mgechev/angular-seed/wiki/Working-with-SASS

I create src/client/scss directory
add main.scss
body {
background-color: red;
}

in project.config.ts
add this.ENABLE_SCSS = true;

same step and scss directory and file

exec npm run build.prod.aot
at angular 5.2.9 commit id df278b87750a5dd17ec24f42aa84a52c34ad18e6
dist/prod/css/main.css generate ok
background-color: red; Take effect

but at angular 6.0.1 commit id b43ccde9f2126cab03d91dd70387d67676db1b45
dist/prod/css/main.css not include background-color: red; css style no any effect

two commit id build success, no any build fail

**Expected behavior**
two commit id build success, include background-color: red; style css

**Minimal reproduction of the problem with instructions**
git clone code
git checkout b43ccde9f2126cab03d91dd70387d67676db1b45
create src/client/scss directory
add main.scss
body {
background-color: red;
}

in project.config.ts
add this.ENABLE_SCSS = true;

exec npm run build.prod.aot

check background-color: red; not effect

**What is the motivation / use case for changing the behavior?**
enable scss build

**Please tell us about your environment:**
Operating system : Mac osx 10.11.6
IDE: VS code 1.24.1
package manager: https://registry.npmjs.org/ npm install
HTTP server: openresty/1.9.7.5

* **Angular Seed Version:** `aaaaf75`
Angular seed version `df278b87750a5dd17ec24f42aa84a52c34ad18e6` OK
Angular seed version `b43ccde9f2126cab03d91dd70387d67676db1b45` FAIL

* **Node:** `node --version` =
`node --version` = v9.10.1

Start with project.config.ts and the SCSS build path described in the issue, then reproduce the problem at commit b43ccde9f2126cab03d91dd70387d67676db1b45 using npm run build.prod.aot. Compare it with df278b87750a5dd17ec24f42aa84a52c34ad18e6; done means dist/prod/css/main.css contains the background-color rule from src/client/scss/main.scss.

Written by the indexing model from the issue text.