Signal an error for File.lastModified on a directory

runtime/bin/file_fuchsia.cc

Index: runtime/bin/file_fuchsia.cc
diff --git a/runtime/bin/file_fuchsia.cc b/runtime/bin/file_fuchsia.cc
index 65d4962678f958f9c0ea376d65f74136ef051761..1c2fe03f0d36aebc68d3de3335a2fcde5a931e0c 100644
--- a/runtime/bin/file_fuchsia.cc
+++ b/runtime/bin/file_fuchsia.cc
@@ -385,6 +385,12 @@ void File::Stat(const char* name, int64_t* data) {
 time_t File::LastModified(const char* name) {
   struct stat st;
   if (NO_RETRY_EXPECTED(stat(name, &st)) == 0) {
+    // Signal an error if it's a directory.
+    if (S_ISDIR(st.st_mode)) {
+      errno = EISDIR;
+      return -1;
+    }
+    // Otherwise assume the caller knows what it's doing.
     return st.st_mtime;
   }
   return -1;