How to force using Tracker (not Zygote) in Flux 0.10

I want to try this for debugging. In fact one of the Flux example codes does not even run under 0.10.1, error mutating arrays not supported.

I tried doing using Tracker before using Flux, however it still gives the error with a stacktrace in Zygote, indicating tracker’s gradient() was not used. The code was originally written before 0.10 and is compatible with Tracker.

How to force the use of Tracker under Flux 0.10?