type WriteFileCallback: make param sourceFiles required. (#18731)
type WriteFileCallback: make param sourceFiles required. (#18731)

Fixes https://github.com/Microsoft/TypeScript/issues/15007
synaptek/TypeScript,Eyas/TypeScript,Microsoft/TypeScript,kpreisser/TypeScript,weswigham/TypeScript,weswigham/TypeScript,nojvek/TypeScript,kitsonk/TypeScript,nojvek/TypeScript,kitsonk/TypeScript,TukekeSoft/TypeScript,donaldpipowitch/TypeScript,RyanCavanaugh/TypeScript,kpreisser/TypeScript,Eyas/TypeScript,kpreisser/TypeScript,Microsoft/TypeScript,basarat/TypeScript,Microsoft/TypeScript,alexeagle/TypeScript,SaschaNaz/TypeScript,donaldpipowitch/TypeScript,TukekeSoft/TypeScript,minestarks/TypeScript,alexeagle/TypeScript,synaptek/TypeScript,RyanCavanaugh/TypeScript,nojvek/TypeScript,kitsonk/TypeScript,donaldpipowitch/TypeScript,RyanCavanaugh/TypeScript,synaptek/TypeScript,microsoft/TypeScript,basarat/TypeScript,donaldpipowitch/TypeScript,SaschaNaz/TypeScript,microsoft/TypeScript,minestarks/TypeScript,basarat/TypeScript,microsoft/TypeScript,alexeagle/TypeScript,SaschaNaz/TypeScript,minestarks/TypeScript,TukekeSoft/TypeScript,nojvek/TypeScript,Eyas/TypeScript,Eyas/TypeScript,SaschaNaz/TypeScript,basarat/TypeScript,synaptek/TypeScript,weswigham/TypeScript