Pin the version of meta used by web_ui (#19544) (#19571)

The latest version of meta now exports "unawaited", which conflicts with the
version of test_api currently used by web_ui.

Co-authored-by: Jason Simmons <jason-simmons@users.noreply.github.com>
diff --git a/lib/web_ui/pubspec.yaml b/lib/web_ui/pubspec.yaml
index 38e0a14..b3e6ff7 100644
--- a/lib/web_ui/pubspec.yaml
+++ b/lib/web_ui/pubspec.yaml
@@ -5,7 +5,7 @@
   sdk: ">=2.9.0-0 <3.0.0"
 
 dependencies:
-  meta: ^1.1.8
+  meta: 1.1.8
 
 dev_dependencies:
   analyzer: 0.39.10