Hi Vadim,
Turns out that unittest is included in the SDK (look under 'dart/dart-sdk/packages' in the trunk build). So you can temporarily use an sdk dependency ('unittest: { sdk: unittest }'), or symlink the unittest/lib/ directory by hand.
Ideally we would push this package to pub, but unfortunately we can't upload a version today without breaking users that install the currently released Editor. Long term we hope it will be possible to upload to pub a package for a particular SDK version, but that's not supported just yet.
Hope this helps.
Cheers,
Siggi