title: Decorator on generator doesn't work
body: <p>I'm trying to create a decorator for a generator function.<br>
  But there is some issue with the declaration.<br>
  What i'm doing wrong?<br></p>
  
  <pre><code>function  nothing (target, key, descriptor) {
    const fn = descriptor.value
    descriptor.value = function (...args) {
      fn.apply(this, args)
    }
  }    
  
  class Test {
    @nothing
    *b(x) {
      const y = yield x
      return y + 1
    }
  }
  </code></pre>
  
  <p>Error:</p>
  
  <pre><code>Unexpected token (36:8)
    34 | class Test {
    35 |   @nothing
  &gt; 36 |   *b(x) {
       |         ^
    37 |     const y = yield "a"
    38 |     return y + 1
    39 |   }
  </code></pre>
  
  <p>Example in babel online:</p>
  
  <p><a href="https://i.stack.imgur.com/j8v5T.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/j8v5T.png" alt=""></a></p>