A small tutorial on how to use the LLVM code generation tool:

mkdir build
cd build
cmake -DTARGET_ARCH=[X64|WASM] ../
make

Suppose that your dart file is example/main.dart, then run:
pkg/vm/tool/release_llvm_precompiler example/main
This will generate a bunch of files in example/:
main.il -- the serialized IL
main.llvm -- the LLVM IR in text format
main.s -- the corresponding assembly
main.S -- the assembly from the Dart AOT pipeline
main.dill
main executable -- the executable from the Dart AOT pipeline
main.so -- the LLVM executable

Then to run this:
./out/ReleaseX64/dart_precompiled_runtime --llvm-mode example/main.so

For Wasm running:
pkg/vm/tool/wasm_precompiler example/main
Produces a WebAssembly module main.wasm, among other things.