js-interop: Specify behaviour of .toString() on non-Dart objects

[rakudrama]

JS-interop should specify how Dart handles the .toString() method for non-Dart objects.
This would ensure some consistency between development and deployment.

[jmesserly]

Excellent question. Ideally, I would like dart:core Object.toString to call JS obj.toString() for JS objects. That way we're behaving "like JS".

We also need to specify the behavior of all of the dart:core Object members (including runtimeType).

As #35952 notes, JavaScript itself does not support .toString() on some objects that don't inherit from Object.prototype, for example Object.create(null).toString(). I'm hesitant to try do different things for JS objects than JS would do. However toString() is very common and developers generally don't expect it to throw (despite the lack of guarantees in Dart or JS), so it may be worth addressing. I'll think more about that part & try to gather some user feedback.

edit: In the particular case of GWT and other JS objects with unusual prototypes, calling JS obj.toString() may be sufficient, assuming they expose that method.