Fix inspector unittests.

sky/framework/inspector/dom-agent.sky

 <script>
 class DOMAgent {
   constructor(delegate) {
     this.enabled = false;
     this.delegate_ = delegate;
     this.nextNodeId_ = 1;
     this.nodeToId_ = new Map();
     this.idToNode_ = new Map();
+    this.observer_ = null;
     Object.preventExtensions(this);
   }
 
   getIdForNode_(node) {
     if (this.nodeToId_.has(node))
       return this.nodeToId_.get(node);
     var id = this.nextNodeId_++;
     this.nodeToId_.set(node, id);
     this.idToNode_.set(id, node);
     return id;
@@ skipping 144 matching lines @@
             node: this.serializeNode_(node),
           });
         }.bind(this));
       }
     }
   }
 }
 
 module.exports = DOMAgent;
 </script>