JFYI, in Angular6 you have to put the conf in the options (in angular.json) :
"serve": {
"builder": "@angular-devkit/build-angular:dev-server",
"options": {
"browserTarget": "app:build",
"ssl": true,
"sslKey": "path to .key",
"sslCert": "path to .crt"
},
...
}