Merge pull request #4683 from pq/1.18.0-dev.2.0

Bump Dart SDK to `1.18.0-dev.2.0`.
diff --git a/bin/cache/dart-sdk.version b/bin/cache/dart-sdk.version
index 511a76e..2dd012e 100644
--- a/bin/cache/dart-sdk.version
+++ b/bin/cache/dart-sdk.version
@@ -1 +1 @@
-1.17.1
+1.18.0-dev.2.0