Run Flutter microbenchmarks in --preview-dart-2 mode (#13225)
Run Flutter microbenchmarks in --preview-dart-2 mode (#13225)

Existing microbenchmarks task is extended instead of adding a new one to consume less horizontal space on Flutter dashboard.

flutter/flutter,flutter/flutter,cbracken/flutter,tvolkert/flutter,tvolkert/flutter,tvolkert/flutter,tvolkert/flutter,flutter/flutter,cbracken/flutter,tvolkert/flutter,flutter/flutter,tvolkert/flutter,Hixie/flutter,tvolkert/flutter,cbracken/flutter,cbracken/flutter,Hixie/flutter,Hixie/flutter,Hixie/flutter,Hixie/flutter,Hixie/flutter,cbracken/flutter,Hixie/flutter,flutter/flutter,Hixie/flutter,flutter/flutter,Hixie/flutter,flutter/flutter,flutter/flutter,flutter/flutter,cbracken/flutter,tvolkert/flutter,cbracken/flutter,tvolkert/flutter,Hixie/flutter,cbracken/flutter,flutter/flutter,tvolkert/flutter,cbracken/flutter,cbracken/flutter