Allow callbacks unioned with null and/or undefined
Allow callbacks unioned with null and/or undefined

basarat/TypeScript,synaptek/TypeScript,alexeagle/TypeScript,RyanCavanaugh/TypeScript,weswigham/TypeScript,basarat/TypeScript,Eyas/TypeScript,chuckjaz/TypeScript,Microsoft/TypeScript,basarat/TypeScript,Eyas/TypeScript,kitsonk/TypeScript,chuckjaz/TypeScript,kpreisser/TypeScript,TukekeSoft/TypeScript,weswigham/TypeScript,nojvek/TypeScript,donaldpipowitch/TypeScript,donaldpipowitch/TypeScript,synaptek/TypeScript,microsoft/TypeScript,SaschaNaz/TypeScript,Microsoft/TypeScript,Microsoft/TypeScript,mihailik/TypeScript,SaschaNaz/TypeScript,kitsonk/TypeScript,kpreisser/TypeScript,donaldpipowitch/TypeScript,kitsonk/TypeScript,synaptek/TypeScript,TukekeSoft/TypeScript,alexeagle/TypeScript,microsoft/TypeScript,SaschaNaz/TypeScript,donaldpipowitch/TypeScript,RyanCavanaugh/TypeScript,minestarks/TypeScript,mihailik/TypeScript,SaschaNaz/TypeScript,nojvek/TypeScript,microsoft/TypeScript,alexeagle/TypeScript,nojvek/TypeScript,RyanCavanaugh/TypeScript,Eyas/TypeScript,kpreisser/TypeScript,minestarks/TypeScript,minestarks/TypeScript,weswigham/TypeScript,synaptek/TypeScript,mihailik/TypeScript,mihailik/TypeScript,chuckjaz/TypeScript,Eyas/TypeScript,chuckjaz/TypeScript,nojvek/TypeScript,basarat/TypeScript,TukekeSoft/TypeScript