[analyzer] Wrong error is reported if an extension type declares an external variable

[sgrekhov]

Analyzer reports an error if an extension type declares an external variable

extension type ET1(int id) {
  external int m; // COMPILE_TIME_ERROR.EXTENSION_TYPE_DECLARES_INSTANCE_FIELD. Extension types can't declare instance fields.
}

According to the spec external instance variables are allowed

A compile-time error occurs if the extension type declaration declares any instance variables, unless they are external.

An external instance variable is just a convenient notation for an external getter and (if not final) an external setter. They are allowed.

Tested on the edge (Aug 24, 2023) SDK on Linux x64

[bwilkerson]

@scheglov

[scheglov]

https://dart-review.googlesource.com/c/sdk/+/322982