title: Using $$ROOT in aggregation order
body: <p>I'm try get data from collection with some order:</p>
  
  <pre><code>db.data.aggregate([
      {$limit: 1000},
      {$group: {
          _id: "$service",
          count: {$sum: 1},
          data: {$push: '$$ROOT'}
      }}
  ]);
  </code></pre>
  
  <p>But get next error:</p>
  
  <pre><code>Error("Printing Stack Trace")@:0
  ()@src/mongo/shell/utils.js:37
  ([object Array])@src/mongo/shell/collection.js:866
  @(shell):6
  
  uncaught exception: aggregate failed: {
      "errmsg" : "exception: FieldPath field names may not start with '$'.",
      "code" : 16410,
      "ok" : 0
  }
  </code></pre>
  
  <p>Where did I go wrong?</p>