Roll engine to 2b1f3dbe25894d7614587f494a25b40fdb344f4a.

This is one before dart roll.
diff --git a/bin/internal/engine.version b/bin/internal/engine.version
index 2c1216d..5ff57c8 100644
--- a/bin/internal/engine.version
+++ b/bin/internal/engine.version
@@ -1 +1 @@
-abd74ed5ed7fdfabb80226db7480ba631f230e0b
+2b1f3dbe25894d7614587f494a25b40fdb344f4a