Fix lint error that was commited

This is what happens when your merge PRs by hand and forget to run the linter. You break everyone's PRs.

src/compiler/tsc.ts

@@ -301,7 +301,7 @@ namespace ts {
             sourceFile.fileWatcher.close();
             sourceFile.fileWatcher = undefined;
             if (removed) {
-                var index = rootFileNames.indexOf(sourceFile.fileName);
+                let index = rootFileNames.indexOf(sourceFile.fileName);
                 if (index >= 0) {
                     rootFileNames.splice(index, 1);
                 }