Fix type for Dart 2 (#18998)
Fix type for Dart 2 (#18998)

Fixes #18995.
flutter/flutter,flutter/flutter,tvolkert/flutter,tvolkert/flutter,tvolkert/flutter,flutter/flutter,flutter/flutter,tvolkert/flutter,Hixie/flutter,tvolkert/flutter,Hixie/flutter,Hixie/flutter,Hixie/flutter,cbracken/flutter,cbracken/flutter,cbracken/flutter,cbracken/flutter,Hixie/flutter,tvolkert/flutter,flutter/flutter,Hixie/flutter,cbracken/flutter,flutter/flutter,cbracken/flutter,flutter/flutter,tvolkert/flutter,tvolkert/flutter,cbracken/flutter,cbracken/flutter,Hixie/flutter,tvolkert/flutter,flutter/flutter,cbracken/flutter,tvolkert/flutter,Hixie/flutter,flutter/flutter,cbracken/flutter,Hixie/flutter,flutter/flutter,Hixie/flutter