[infra] Set up owners for pubspec.yaml

Change-Id: Idf53a0934eee3d77c7799ca4eaaf90af989ce015
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/425840
Reviewed-by: Ivan Inozemtsev <[email protected]>
Reviewed-by: Alexander Thomas <[email protected]>

Author: dcharkes

OWNERS

@@ -10,6 +10,8 @@ [email protected] #{LAST_RESORT_SUGGESTION}
 
 # DEPS
 per-file DEPS=file:/tools/OWNERS_ENG
+per-file pubspec.yaml=file:/tools/OWNERS_ENG
+
 # Updates fuchsia components only.
 per-file DEPS=file:/tools/OWNERS_FUCHSIA