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

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

Author: jason-simmons

lib/web_ui/pubspec.yaml

@@ -5,7 +5,7 @@ environment:
   sdk: ">=2.9.0-0 <3.0.0"
 
 dependencies:
-  meta: ^1.1.8
+  meta: 1.1.8
 
 dev_dependencies:
   analyzer: 0.39.10