UsingDomains() assigned process_tickCallback when it had already
been set by MakeCallback() a few frames down the call stack.
Dispose the handle first or we'll retain whatever is in the lexical
closure of the old process._tickCallback function.
Fixes #5795.
Local<Function> ndt = ndt_v.As<Function>();
process->Set(String::New("_tickCallback"), tdc);
process->Set(String::New("nextTick"), ndt);
+ process_tickCallback.Dispose(); // Possibly already set by MakeCallback().
process_tickCallback = Persistent<Function>::New(tdc);
return Undefined();
}