Add AssertUIGoroutine to the Driver interface.

[ben-clayton]

This is can be used to assert that a function is being called on the UI
go-routine, which is expected for all of the controls.

AssertUIGoroutine is called from the two most commonly called internal
functions of controls: Redraw and Relayout.

Should catch race conditions in applications using GXUI.

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[ben-clayton]

Pretty horrible - but has caught issues in some projects that uses GXUI.
Opening as a pull request to hear thoughts from anyone who might have opinions on this.

[sunwangme]

very good

[warpfork]

Sounds reasonable to me. Defense in depth and rapid detection 👍

[dmitshur]

@ben-clayton, I don't think runtime.Callers is completely implemented in GopherJS yet, so this will currently fail there:

Is it okay if I made a PR here to disable this debug feature under js build tag, until it is supported?