Bump acorn from 5.7.3 to 5.7.4 in /src/app-engine/SPA-websocket/client

Bumps [acorn](https://github.com/acornjs/acorn) from 5.7.3 to 5.7.4. - [Release notes](https://github.com/acornjs/acorn/releases) - [Commits](acornjs/acorn@5.7.3...5.7.4) Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

src/app-engine/SPA-websocket/client/yarn.lock

@@ -1019,8 +1019,8 @@ acorn-walk@^6.0.1:
  resolved "https://registry.yarnpkg.com/acorn-walk/-/acorn-walk-6.1.1.tgz#d363b66f5fac5f018ff9c3a1e7b6f8e310cc3913"
 
 acorn@^5.0.0, acorn@^5.5.3, acorn@^5.6.2:
- version "5.7.3"
- resolved "https://registry.yarnpkg.com/acorn/-/acorn-5.7.3.tgz#67aa231bf8812974b85235a96771eb6bd07ea279"
+ version "5.7.4"
+ resolved "https://registry.yarnpkg.com/acorn/-/acorn-5.7.4.tgz#3e8d8a9947d0599a1796d10225d7432f4a4acf5e"
 
 acorn@^6.0.1, acorn@^6.0.2, acorn@^6.0.4:
  version "6.0.5"