Use Directory.systemTemp getter in pkg subdirectory of repository.

pkg/watcher/test/utils.dart

Index: pkg/watcher/test/utils.dart
diff --git a/pkg/watcher/test/utils.dart b/pkg/watcher/test/utils.dart
index 656ddf8edb2f5ad55c17e073ec4af3f02c5f57b8..28d57b05c61e708c499190a5906076739eb87561 100644
--- a/pkg/watcher/test/utils.dart
+++ b/pkg/watcher/test/utils.dart
@@ -46,7 +46,7 @@ void initConfig() {
 ///
 /// This should usually be called by [setUp].
 void createSandbox() {
-  var dir = new Directory("").createTempSync();
+  var dir = Directory.systemTemp.createTempSync('watcher_test_');
   _sandboxDir = dir.path;
 
   _mockFileModificationTimes = new Map<String, int>();