Bump acorn from 6.3.0 to 6.4.2 in /src

Bumps [acorn](https://github.com/acornjs/acorn) from 6.3.0 to 6.4.2.
- [Release notes](https://github.com/acornjs/acorn/releases)
- [Commits](https://github.com/acornjs/acorn/compare/6.3.0...6.4.2)

Signed-off-by: dependabot[bot] <support@github.com>

src/package-lock.json

@@ -205,9 +205,9 @@
       "dev": true
     },
     "acorn": {
-      "version": "6.3.0",
-      "resolved": "https://registry.npmjs.org/acorn/-/acorn-6.3.0.tgz",
-      "integrity": "sha512-/czfa8BwS88b9gWQVhc8eknunSA2DoJpJyTQkhheIf5E48u1N0R4q/YxxsAeqRrmK9TQ/uYfgLDfZo91UlANIA==",
+      "version": "6.4.2",
+      "resolved": "https://registry.npmjs.org/acorn/-/acorn-6.4.2.tgz",
+      "integrity": "sha512-XtGIhXwF8YM8bJhGxG5kXgjkEuNGLTkoYqVE+KMR+aspr4KGYmKYg7yUe3KghyQ9yheNwLnjmzh/7+gfDBmHCQ==",
       "dev": true
     },
     "ajv": {