fix: Unpatched builds are slower than patched builds on iOS

[eseidel]

Originally when implementing mixed-mode, we decided that the "off" behavior should be to run everything within the interpreter to minimize surprises for customers of their app running much slower after patching.

Of course, now that we've made mixed mode fast(er) now patching with an empty patch can end up causing the app to run much faster than unpatched.

#674

This is silly, and should be relatively easy to fix in our Dart runtime. It will then make it the case that patching can cause a slowdown of your app, but since that slowdown should be much smaller than before that seems like the correct behavior.

[eseidel]

In all cases, patched and unpatched builds should now be faster than they were with our old iOS engine (Dec 2023 and before). This bug is about our eventual desire to change unpatched builds to run everything on the CPU (thus making them 100% full speed, but risking users being surprised when patched builds may be slower than unpatched builds).

[eseidel]

https://github.com/shorebirdtech/dart-sdk/issues/284 was filed to cover some of the details of making this work.

[eseidel]

Felix wired this up just now, and will be out in our next release (later today).