fix(schematics): add parse5 dep

package.json

@@ -41,6 +41,9 @@
     "tslib": "^1.9.0",
     "zone.js": "^0.8.26"
   },
+  "optionalDependencies": {
+    "parse5": "^5.0.0"
+  },
   "devDependencies": {
     "@angular-devkit/core": "^0.5.12",
     "@angular-devkit/schematics": "^0.5.12",

src/lib/schematics/utils/html.ts

@@ -10,6 +10,10 @@ import {Project} from './devkit-utils/config';
  * @param src the src path of the html file to parse
  */
 export function getHeadTag(host: Tree, src: string) {
+  if (!parse5) {
+    throw new SchematicsException('parse5 depedency not found! Please install parse5 from npm to continue.');
+  }
+
   const document = parse5.parse(src,
     {locationInfo: true}) as parse5.AST.Default.Document;