Views tests should use the 'views' experimental flag

[johnniwinther]

The views tests, for instance co19/LanguageFeatures/Views/syntax_A02_t04.dart, use the extension-types flag:

// SharedOptions=--enable-experiment=extension-types

instead of the correct views flag:

// SharedOptions=--enable-experiment=views

[eernstg]

The mechanism was just renamed to 'inline class'. Perhaps it would be helpful to rename the flag, too?

[johnniwinther]

Ok. I (try to) rename the flag (haven't tried that before) and give a heads up when that change has landed.