Add a very minimal CSSAgent

sky/framework/inspector/dom-agent.sky

 <script>
 function DOMAgent(delegate) {
   this.enabled = false;
   this.delegate_ = delegate;
   this.nextNodeId_ = 1;
   this.nodeToId_ = new Map();
   this.idToNode_ = new Map();
 }
 
 DOMAgent.prototype.getIdForNode_ = function(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;
 };
 
+DOMAgent.prototype.getNodeForId = function(nodeId) {
+  return this.idToNode_.get(nodeId);
+};
+
 DOMAgent.prototype.serializeChildren_ = function(node) {
   var children = [];
   for (var child = node.firstChild; child; child = child.nextSibling) {
     var record = this.serializeNode_(child);
     if (record)
       children.push(record);
   }
   return children;
 };
 
@@ skipping 127 matching lines @@
           previousNodeId: previousNodeId,
           node: this.serializeNode_(node),
         });
       }.bind(this));
     }
   }
 };
 
 this.exports = DOMAgent;
 </script>