build: speed-up stylesheet building (#12426)

* Switches back from `dart-sass` to `node-sass` due to a pretty huge slow-down we noticed (can be revisited)
* No longer copies all SCSS files to the temporary `package` output just in favor of the demo-app (slows down unnecessarily). This now allows us to live-reload theme changes within a second.
* No longer rebuilds all package SCSS files twice for the ES5 output (rel: https://github.com/angular/material2/pull/7403)

package.json

@@ -84,7 +84,6 @@
     "gulp-cli": "^1.3.0",
     "gulp-connect": "^5.0.0",
     "gulp-conventional-changelog": "^1.1.3",
-    "gulp-dart-sass": "^0.9.0",
     "gulp-dom": "^0.9.17",
     "gulp-flatten": "^0.3.1",
     "gulp-highlight-files": "^0.0.5",
@@ -92,6 +91,7 @@
     "gulp-if": "^2.0.2",
     "gulp-markdown": "^1.2.0",
     "gulp-rename": "^1.2.2",
+    "gulp-sass": "^3.1.0",
     "gulp-transform": "^2.0.0",
     "gulp-util": "^3.0.8",
     "hammerjs": "^2.0.8",
@@ -111,6 +111,7 @@
     "minimatch": "^3.0.4",
     "minimist": "^1.2.0",
     "moment": "^2.18.1",
+    "node-sass": "^4.5.3",
     "parse5": "^5.0.0",
     "protractor": "^5.2.0",
     "request": "^2.83.0",
@@ -119,7 +120,6 @@
     "rollup-plugin-alias": "^1.4.0",
     "rollup-plugin-node-resolve": "^3.0.3",
     "run-sequence": "^1.2.2",
-    "sass": "^1.10.1",
     "scss-bundle": "^2.0.1-beta.7",
     "selenium-webdriver": "^3.6.0",
     "sorcery": "^0.10.0",