Update the grammar to prevent an extension from having the name type

[eernstg]

This PR adjusts the Dart grammar such that the name of an extension declaration can not be type, without turning type into a built-in identifier. This eliminates an ambiguity where the same declaration could at the same time be an extension type declaration and an extension declaration. See #3258 for more details.

[lrhn]

LGTM.

Is this a breaking change or a language versioned change?
That is, will it affect language versions prior to releasing extension types?
(I'm fine with it being breaking.)

If not breaking, it should probably not go into the spec before the rest of extension types.

[eernstg]

It is a breaking change at this time or later, because we could have a declaration of the form extension type on (T v,) {...} which is currently an extension named type. That declaration would be a syntax error unless and until the extension type feature is enabled, and it would then surely be have some compile-time errors because the declaration would need to use this differently when it is parsed as an extension type. Also, every call-site for members declared by this extension would almost inevitably turn into compile-time errors.

Nevertheless, I cannot see any occurrences of extension type on anywhere, and it seems likely that there are no such declarations in the first place.

So it's a breaking change no matter what, and we might as well deal with it now.

[eernstg]

OK, I initiated the breaking change process: dart-lang/sdk#53883.