Fix off-by-one bug in many_files_test.

sdk/lib/_internal/pub/test/lish/many_files_test.dart

Index: sdk/lib/_internal/pub/test/lish/many_files_test.dart
diff --git a/sdk/lib/_internal/pub/test/lish/many_files_test.dart b/sdk/lib/_internal/pub/test/lish/many_files_test.dart
index 1adec26fa570943988428aa9719c823f8d00103e..776779aee1aba4aa9629cf6ce4df3899dd1ac58f 100644
--- a/sdk/lib/_internal/pub/test/lish/many_files_test.dart
+++ b/sdk/lib/_internal/pub/test/lish/many_files_test.dart
@@ -19,10 +19,14 @@ import 'utils.dart';
 /// The maximum number of bytes in an entire path.
 ///
 /// This is [Windows's number][MAX_PATH], which is a much tighter constraint
-/// than OS X or Linux. We use it everywhere for consistency.
+/// than OS X or Linux. We subtract one because Windows counts it as the number
+/// of bytes in a path C string including the terminating NUL but we only count
+/// characters here.
+///
+/// We use this limit on all platforms for consistency.
 ///
 /// [MAX_PATH]: https://msdn.microsoft.com/en-us/library/windows/desktop/aa383130(v=vs.85).aspx
-const _pathMax = 260;
+const _pathMax = 260 - 1;
 
 main() {
   initConfig();