add missing type annotation

src/compiler/program.ts

@@ -2187,7 +2187,7 @@ namespace ts {
         }
 
         function createRedirectSourceFile(redirectTarget: SourceFile, unredirected: SourceFile, fileName: string, path: Path, resolvedPath: Path, originalFileName: string): SourceFile {
-            const redirect = Object.create(redirectTarget);
+            const redirect: SourceFile = Object.create(redirectTarget);
             redirect.fileName = fileName;
             redirect.path = path;
             redirect.resolvedPath = resolvedPath;