Documentation should mention Isolate.spawn() doesn't work with dart:html apps

[DartBot]

This issue was originally filed by [email protected]

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URL (if available):
https://api.dartlang.org/apidocs/channels/stable/#dart-isolate.Isolate

Issue/feedback:

    There's no mention of the fact that Isolate.spawn() doesn't work with dart:html apps.

[lrhn]

Could we make Isolate.spawn work in such a way that any DOM access will throw (e.g., new StateError("concurrent access")), but the rest of the spawning will still work?

That will allow a library to use Isolate.spawn without knowing if it is in the DOM isolate, and since it doesn't know, it likely won't use the DOM anyway.

[lrhn]

Added Duplicate label.
Marked as being merged into #16209.