仕様: https://github.com/wycats/javascript-decorators

プロパティ自体に decorator は付か無いので、しかたなく setter を作ったけど、なんかうまくできないだろうか。
(この使い方自体が筋悪な気もするが、 decorator でやりたいのってこういうのな気がする。)

// $ babel-node --optional es7.decorators index.js
class Assert {
  private(target, name, descriptor) {
    let value;
    descriptor.set = (val) => {
      value = val;
    }
    descriptor.get = () => {
      return value;
    }
  }

  required(target, name, descriptor) {
    let original = descriptor.set;
    descriptor.set = (val) => {
      console.assert(val !== undefined, `${name} is required`);
      original(val);
    }
  }

  validate(target, name, descriptor) {
    let original = descriptor.set;
    descriptor.set = (val) => {
      console.assert(val.method !== undefined, `${name}.val is required`);

      let method_list = ['get', 'post', 'put', 'delete'];
      console.assert(method_list.indexOf(val.method) > -1, `${name}.val should be one of ${method_list} but ${val.method}`);
      original(val);
    }
  }
}

var assert = new Assert();

class Fetcher {

  @assert.required
  @assert.private
  set url(val) {}

  @assert.validate
  @assert.required
  @assert.private
  set option(val) {}

  constructor(url, option) {
    this.url = url;
    this.option = option;
  }
}

var fetcher = new Fetcher('http://example.com', { method: 'get' });
console.log(fetcher);         // {}
console.log(fetcher.url);     // http://example.com
console.log(fetcher.options); // { method: 'get' }

追記

関数の引数を先に奪ってバリデート

class Fetcher {

  @assert.required
  @assert.private
  set url(val) {}

  @assert.validate
  @assert.required
  @assert.private
  set option(val) {}

  constructor(url, option) {
    this.url = url;
    this.option = option;
  }
}

var fetcher = new Fetcher('http://example.com', { method: 'get' });
console.log(fetcher);         // {}
console.log(fetcher.url);     // http://example.com
console.log(fetcher.options); // { method: 'get' }

汎用の関数 wrapper を作るアノテーションを別途用意。

class Fetcher {

  @assert.required
  @assert.private
  set url(val) {}

  @assert.validate
  @assert.required
  @assert.private
  set option(val) {}

  constructor(url, option) {
    this.url = url;
    this.option = option;
  }
}

var fetcher = new Fetcher('http://example.com', { method: 'get' });
console.log(fetcher);         // {}
console.log(fetcher.url);     // http://example.com
console.log(fetcher.options); // { method: 'get' }