Fix dart:js for cross-frame Arrays

[sigmundch]

the check o instanceof Array here:
https://github.com/dart-lang/sdk/blob/master/sdk/lib/js/dart2js/js_dart2js.dart#L153

does not work when the array came from a different iframe.

@rakudrama suggests we should use Array.isArray instead.

[LorenVS]

I believe this check also needs to be updated:

sdk/sdk/lib/js/dart2js/js_dart2js.dart

Line 628 in f0babb6

if (JS('bool', '# instanceof Array', o)) {

[jmesserly]

FYI, using "dart:js" JsObject is deprecated (not sure if it has been marked as such, but it should be). I'd be interested to understand the use case and try to find a better solution for you?

"package:js" is the supported approach (https://pub.dartlang.org/packages/js), there are examples here: https://github.com/matanlurey/dart_js_interop, and lots of improvements planned (#35084)

[jmesserly]

A related issue, apparently dart2js interceptor dispatch uses a.constructor == JsArray (?), which means it doesn't recognize generic arrays created by DDC (except List<dynamic>, presumably). This is based on user reports.

(Aside: I wonder if we'd eventually want some sort of Dart-to-Dart interop for multi-app scenarios, rather than rely on JS interop for communication between the Dart apps.)