[Extension type] Core library updates

[itsjustkevin]

[shilangyu]

Where could one read more about this Views feature? Unfortunately I could not find it in the language repo. Is that a library-only change?

[mraleph]

@shilangyu you can check these two proposals to get the idea:

https://github.com/dart-lang/language/blob/master/working/1426-extension-types/feature-specification-views.md

https://github.com/dart-lang/language/blob/master/working/extension_structs/overview.md

tldr is that views provide a way to define a non-reified type, which only exists as a separate entity during compilation and is erased in runtime. It's a zero-overhead to provide a wrapper (with its own helper methods) around an existing representation type.

[eernstg]

Please note that this feature has been renamed: It is now the 'inline class' feature.