In the two experiments that we did LLVM brought only marginal benefits so we could not warrant the huge dependency and associated maintenance costs. We already have a good compilation pipeline, which was developed for the JIT mode and we use that for AOT with good results. Adding LLVM on top increases complexity - suddenly to tweak things you need to be an expert both in our compilation pipeline and LLVM (which is probably 100x larger than the whole Dart VM source code).

A lot of optimizations which benefit Dart code size and performance require high level optimizations anyway, so having LLVM does not help you in any way.

That said there are obvious benefits for having LLVM as a backend - so we are planning to explore it yet again in the near future.