Fix #9843. IScriptSnapshot can return undefined

Author: HerringtonDarkholme

src/services/services.ts

@@ -94,7 +94,7 @@ namespace ts {
          * change range cannot be determined.  However, in that case, incremental parsing will
          * not happen and the entire document will be re - parsed.
          */
-        getChangeRange(oldSnapshot: IScriptSnapshot): TextChangeRange;
+        getChangeRange(oldSnapshot: IScriptSnapshot): TextChangeRange | undefined;
 
         /** Releases all resources held by this script snapshot */
         dispose?(): void;