In emerging flow-pomodoro, I found that it was necessary to emerge the following dependency versions: dev-qt/qtdeclarative-5.5.1 dev-qt/qtquickcontrols-5.5.1 dev-qt/qtquick1-5.5.1
I successfully merged this without issue on Qt 5.4.x. I suspect your issue was a mix of Qt 5.4.x and 5.5.x. As such, I'm going to close this bug, but feel free to reopen if you have information to the contrary (Logs would be nice if you decide to re-open). Also, from upstream: https://github.com/iamsergio/flow-pomodoro/blob/master/distros/archlinux/PKGBUILD#L12 makedepends=('qt5-base>=5.4.0' 'qt5-declarative>=5.4.0' 'qt5-quickcontrols>=5.4.0')
