<rant incoming, be warned>
I'll use the Mario example from one of his first widely disseminated talks as an example: In the Mario example you see him run time backwards. That's actually reasonably achievable by just recording all previous states or (more plausibly) all state changes and then "just" replaying backwards. Then he runs time forwards again after having changed a setting (or some such). This is, again achievable within limits, see e.g. "rr".
The problem comes when he shows a projected future path of Mario. Because of a) Turing Completeness (TC) + the Halting problem, and b) the fact that much code actually interacts with other systems which are not able to just rewind (and/or themselves interact with systems that ...).
The only plausible scenario in which this is even remotely possible in practice is if the programs adhere to strict rules about side effects, have built-in "project into the future" (or have no side effects so that we can run them to see what the results are _without affecting anything external_.). That is, it's not about fancy IDEs -- it's about programming languages and turning away from Turing Completeness and/or non-abstract/non-formalized effects. FRP/(React+Flux) sort of work for this if you adhere strictly to purity, but then again break down when it comes to things like "what is the focused element" if the user clicks around when you're replaying.
I don't want to be a party-pooper, but good luck changing the programming paradigm of everyone to achieve this vision. For it to work everything has to work in this FRPish way, and that's just never going to happen -- for example, networks (for real-world reasons) cannot behave this way, etc. etc. I WISH we could have everything behave this way, but we can't -- billions of e.g. browser installations which have to have backward-compatible DOM implementations and millions (billions?) of installations of C/C++-based operating systems say so.
Now, I'm not against Imagining Things How They Could Be, but if there's mathematical proof that "thing X" is not possible one either needs to explain how you are not doing "thing X" (perhaps not using a TC language, or disallowing non-pure state) or shut up. Brett Victor and many "visionaries" like him are just very bad at stating their fundamental assumptions up front (which is perhaps part of the reason we don't get to see the code, it's probably just simple pure FRP-like code) so that we can evaluate the vision against the assumptions. (Light Table just seemed to assume that the assumptions were plausible and see where that got them. Funded but with very little actual technological advancement to show for it.)
... and also don't get me started on the whole "Kill Math" thing. Ugh.
(Sorry about the rantish nature of this -- I should probably just write this up properly somewhere so I could properly flesh it out. I just get tired of this "What A Genius!" and "Well, it would take too much time away from inspiration to actually make the thing!" credulous nonsense.)