Update .gitignore to new `dart_tool` pub cache (dart-lang/stack_trace#40)

See dart-lang/sdkdart-lang/stack_trace#32030
diff --git a/pkgs/stack_trace/.gitignore b/pkgs/stack_trace/.gitignore
index 7dbf035..34595d7 100644
--- a/pkgs/stack_trace/.gitignore
+++ b/pkgs/stack_trace/.gitignore
@@ -1,15 +1,6 @@
+# See https://www.dartlang.org/guides/libraries/private-files
 # Don’t commit the following directories created by pub.
-.buildlog
-.pub/
-build/
-packages
+.dart_tool/
 .packages
-
-# Or the files created by dart2js.
-*.dart.js
-*.js_
-*.js.deps
-*.js.map
-
-# Include when developing application packages.
-pubspec.lock
\ No newline at end of file
+.pub/
+pubspec.lock