fix(property-descriptor): do not fail for events without targets
fix(property-descriptor): do not fail for events without targets

See #323

angular/zone.js,mlaval/zone.js,richardmurri/zone.js,angular/zone.js,mhevery/zone.js,mlaval/zone.js,mhevery/zone.js,juliemr/zone.js,juliemr/zone.js,juliemr/zone.js,mhevery/zone.js,richardmurri/zone.js,mhevery/zone.js,juliemr/zone.js,richardmurri/zone.js,mlaval/zone.js,mlaval/zone.js,angular/zone.js,angular/zone.js,richardmurri/zone.js,angular/zone.js