feat(build): added postcss-discard-comments

Close angular#2593

Author: grizzm0

package.json

@@ -84,6 +84,7 @@
     "offline-plugin": "^3.4.1",
     "opn": "4.0.1",
     "parse5": "^2.1.5",
+    "postcss-discard-comments": "^2.0.4",
     "postcss-loader": "^0.9.1",
     "protractor": "^3.3.0",
     "raw-loader": "^0.5.1",

packages/angular-cli/models/webpack-build-production.ts

@@ -43,7 +43,10 @@ export const getWebpackProdConfigPartial = function(projectRoot: string, appConf
               [/\[?\(?/, /(?:)/]
             ],
             customAttrAssign: [/\)?\]?=/]
-          }
+          },
+          postcss: [
+            require('postcss-discard-comments')
+          ]
         }
       })
     ],