This is the major thing holding me back from using Deno more at the moment.
Not being able to mock out a class/function or spy on a particular call etc means that there are large parts of my code/particular situations that are not able to be tested easily without refactoring my entire codebase to use manual IoC and excessive-OO to hide things away under layers of interfaces etc. E.g. I cannot currently mock out `Deno.connect` (....? or can I?) so it is hard to test all sitautions there. Unfortunately this sort of thing (connecting to a remote system) is often quite critical and would benefit hugely from extensive testing.
It would be great to be able to create tests where calls to Deno.connect throw errors, or where it returns 0 bytes, or a special sequence of bytes and so on and so on to verify that my code works - this is the classic sort of `Spy` functionality seen and used extensively in Jasmine et al.
I know there are third-party solutions to this, but it would be nice to see this in the standard distribution.