Javascript handlers are always in the root zone

[johnpryan]

Description

Steps to Reproduce

Initialize polymer in a zone:

main() {
  runZoned(() async {
    await initPolymer();
  });
}

Expected Results

a Polymer component handle (e.g. on-tap="tapHandler") should run in the same zone that called await initPolymer(): printing Zone.current should be a CustomZone, not the root zone.

Actual Results

The root zone

[johnpryan]

This seems related to dart-lang/sdk#17301

[johnpryan]

It's also worth noting that the initPolymer() API used to return the zone in 0.16.0