Clean up the semantics of io.dart operations w.r.t. symlinks.

utils/pub/system_cache.dart

Index: utils/pub/system_cache.dart
diff --git a/utils/pub/system_cache.dart b/utils/pub/system_cache.dart
index b6467b5f88f78f12aa18e4928c1ea1fce2ec7007..1b30ea8501904819265c59c25b194eb0bb48b70c 100644
--- a/utils/pub/system_cache.dart
+++ b/utils/pub/system_cache.dart
@@ -111,7 +111,6 @@ class SystemCache {
   /// Deletes the system cache's internal temp directory.
   void deleteTempDir() {
     log.fine('Clean up system cache temp directory $tempDir.');
-    if (!dirExists(tempDir)) return;
-    deleteDir(tempDir);
+    if (dirExists(tempDir)) deleteEntry(tempDir);
   }
 }