Re-enable Object.observe and add enforcement for security invariants.

src/messages.js

Index: src/messages.js
diff --git a/src/messages.js b/src/messages.js
index bea681acacaef77ec417e478b3704a65044bd0ff..1965da104e33106d07fcf2de881f72363c9cddda 100644
--- a/src/messages.js
+++ b/src/messages.js
@@ -77,6 +77,7 @@ var kMessages = {
   observe_perform_non_string:    ["Invalid non-string changeType"],
   observe_perform_non_function:  ["Cannot perform non-function"],
   observe_notify_non_notifier:   ["notify called on non-notifier object"],
+  observe_global_proxy:          ["%0", " cannot be called on the global proxy object"],
   not_typed_array:               ["this is not a typed array."],
   invalid_argument:              ["invalid_argument"],
   data_view_not_array_buffer:    ["First argument to DataView constructor must be an ArrayBuffer"],