Download and install the Dart source tree using the standard instructions for building Dart.
To build for Fuchsia, you must first update your `.gclient file with:
"custom_vars": { "download_fuchsia_deps": True, },
./tools/build.py --mode=release --os=fuchsia --arch=arm64 create_sdk runtime
./tools/test.py -nvm-fuchsia-release-arm64 -j4 ffi