Fix text_style_text.dart to be Dart 2 compliant. (#14559)
Fix text_style_text.dart to be Dart 2 compliant. (#14559)

In Dart 2 mode we throw different exception: compiler generates
throw of a compilation error instead of runtime throwing
noSuchMethod.
flutter/flutter,cbracken/flutter,flutter/flutter,tvolkert/flutter,flutter/flutter,tvolkert/flutter,flutter/flutter,tvolkert/flutter,Hixie/flutter,flutter/flutter,cbracken/flutter,Hixie/flutter,flutter/flutter,Hixie/flutter,cbracken/flutter,Hixie/flutter,Hixie/flutter,Hixie/flutter,flutter/flutter,Hixie/flutter,cbracken/flutter,tvolkert/flutter,cbracken/flutter,flutter/flutter,cbracken/flutter,tvolkert/flutter,tvolkert/flutter,tvolkert/flutter,Hixie/flutter,tvolkert/flutter,tvolkert/flutter,Hixie/flutter,cbracken/flutter,tvolkert/flutter,Hixie/flutter,cbracken/flutter,cbracken/flutter,flutter/flutter,cbracken/flutter,flutter/flutter