Add --no-preview-dart-2 when in dart1 mode. (#17708)
Add --no-preview-dart-2 when in dart1 mode. (#17708)

This is follow-up to https://github.com/flutter/flutter/pull/17682 to fix dart1 tests.
Hixie/flutter,tvolkert/flutter,tvolkert/flutter,cbracken/flutter,cbracken/flutter,flutter/flutter,flutter/flutter,tvolkert/flutter,Hixie/flutter,flutter/flutter,cbracken/flutter,tvolkert/flutter,Hixie/flutter,tvolkert/flutter,tvolkert/flutter,tvolkert/flutter,cbracken/flutter,Hixie/flutter,Hixie/flutter,Hixie/flutter,flutter/flutter,cbracken/flutter,flutter/flutter,flutter/flutter,flutter/flutter,tvolkert/flutter,Hixie/flutter,flutter/flutter,Hixie/flutter,cbracken/flutter,tvolkert/flutter,cbracken/flutter,tvolkert/flutter,cbracken/flutter,cbracken/flutter,flutter/flutter,Hixie/flutter,Hixie/flutter,cbracken/flutter,flutter/flutter