code: private FlowScope tightenTypeAfterDereference(Node n, FlowScope scope) {
      if (n.isQualifiedName()) {
        JSType type = getJSType(n);
        JSType narrowed = type.restrictByNotNullOrUndefined();
        if (!type.equals(narrowed)) {
          scope = narrowScope(scope, n, narrowed);
        }
      }
      return scope;
    }
docs: If we access a property of a symbol, then that symbol is not
  null or undefined.