[RN Debugger] Don't try to handle messages without a method

Summary:
Some messages are special and are intended for the devtools, like `{$open: id}` and `{$error: id}`. The main debugger-ui page can't handle these and thinks something is wrong when `object.method` is undefined. This diff handles messages only if they specify a method.

Fixes #2377

Closes https://github.com/facebook/react-native/pull/2405
Github Author: James Ide <ide@jameside.com>

packager/debugger.html

@@ -85,6 +85,10 @@ function connectToDebuggerProxy() {
 
   ws.onmessage = function(message) {
     var object = JSON.parse(message.data);
+    if (!object.method) {
+      return;
+    }
+
     var sendReply = function(result) {
       ws.send(JSON.stringify({replyID: object.id, result: result}));
     };