ReSub: Compiler error using @key decorator

I have tested the simpler subscription examples successfully, and now I am trying autosubscriptions using the @key decorator, such as in your example:

@autoSubscribe
    getTodosForUser(@key username: string) {
        return this._todosByUser[username];
    }

Unfortunately, I get a compiler error:

Error: Module parse failed: Unexpected character ‘@’ (1:20609) You may need an appropriate loader to handle this file type.

It seems like Babel has a problem with the @key decorator being used within a function, and not with other decorators like @autoSubscribe and @AutoSubscribeStore.

I’ve checked my babel.config.js and everything seems OK, using:

  const plugins = [
    ['@babel/plugin-proposal-decorators', { legacy: true }],
    ["@babel/plugin-proposal-class-properties", { "loose": true}]
  ];

Did someone else come across this problem? Any recommendation on how to solve this?