PR feedback and clean up.

Author: Paul van Brenk

src/services/services.ts

@@ -1347,10 +1347,8 @@ namespace ts {
         function getNavigateToItems(searchValue: string, maxResultCount?: number, fileName?: string): NavigateToItem[] {
             synchronizeHostData();
 
-            const sourceFiles = !fileName ? program.getSourceFiles() : [program.getSourceFile(fileName)];
-
-            const checker = getProgram().getTypeChecker();
-            return ts.NavigateTo.getNavigateToItems(sourceFiles, checker, cancellationToken, searchValue, maxResultCount);
+            const sourceFiles = fileName ? [getValidSourceFile(fileName)] : program.getSourceFiles();
+            return ts.NavigateTo.getNavigateToItems(sourceFiles, program.getTypeChecker(), cancellationToken, searchValue, maxResultCount);
         }
 
         function getEmitOutput(fileName: string): EmitOutput {