dart:html IntersectionObserver api requires a call to dart:js allowInterop on dart2js

[jacob314]

I'm guessing we are just missing a call to convert a Dart function into a JS function in the dart:html code generation.

As expected, IntersectionObserver works under DDC where functions can be passed between Dart and JS without conversion.

Example where I had to add allow interop:
flutter/devtools@13c304c

[terrylucas]

Fix in CL