Up until recently due to some issues with build scripts and MSVC parameters our bots building 64-bit SDK on Windows were just silently building 32-binaries. I have fixed[1] it couple of weeks ago, but my fix did not make it into 1.5. Sadly it's not just a build script fix, but also some changes in Dart VM to accommodate for ABI differences, so we decided not to merge. Next release will have 64-bit dart binary on Windows. Bleeding edge SDKs already contain correct version of dart binary.

[1] https://code.google.com/p/dart/source/detail?r=37078