Fix inspector unittests.

BUG=
R=ojan@chromium.org

Review URL: https://codereview.chromium.org/821643007

sky/framework/inspector/dom-agent.sky

diff --git a/sky/framework/inspector/dom-agent.sky b/sky/framework/inspector/dom-agent.sky
index 36a3778..0f78c0d 100644
--- a/sky/framework/inspector/dom-agent.sky
+++ b/sky/framework/inspector/dom-agent.sky
@@ -6,6 +6,7 @@
     this.nextNodeId_ = 1;
     this.nodeToId_ = new Map();
     this.idToNode_ = new Map();
+    this.observer_ = null;
     Object.preventExtensions(this);
   }