Do not gitignore workspace pubspec.lock (#9561)

Author: elliette

.gitignore

@@ -4,7 +4,10 @@
 .dart_tool/
 .packages
 .pub/
-pubspec.lock
+
+# Exclude pubspec.lock files in tests and example fixtures.
+packages/devtools_app/test/**/pubspec.lock
+packages/devtools_extensions/example/**/pubspec.lock
 
 .idea/
 .data_dir/