add @empty support for pluginLoader
add @empty support for pluginLoader

systemjs/builder,alexisvincent/builder,alexisvincent/builder,ahmedshuhel/builder,jrauschenbusch/builder,robwormald/builder,richardTowers/builder,alexandr2110pro/builder,richardTowers/builder,Psvensso/builder,whitneyit/builder,alexandr2110pro/builder,tonytanger/builder,jrauschenbusch/builder,tonytanger/builder,lookfirst/builder,buddhike/builder,douglasduteil/builder,matthewbauer/builder,lookfirst/builder,Psvensso/builder,ahmedshuhel/builder,systemjs/builder,alexisvincent/builder,Munter/builder,JeroenVinke/builder,Munter/builder,buddhike/builder,synaptek/builder,synaptek/builder,douglasduteil/builder,matthewbauer/builder,JeroenVinke/builder,robwormald/builder,systemjs/builder,whitneyit/builder,jrauschenbusch/builder