Implicit call method tear-offs with extension typed receivers

[eernstg]

The proposal in dart-lang/language#3467 was accepted by the language team. This means that it should be possible to perform .call method tear-offs also in the case where the receiver is an expression whose static type is an extension type. In this case it should be possible to tear off both a call method which is a non-extension type member (that is, inherited from some "normal" type), and also a call method which is an extension type member (from the receiver type or "inherited" from some other extension type).

@sgrekhov, I think no such tests have been written, isn't that true?

[sgrekhov]

We have a couple of tests that dealing with a call() method but not in the way described in dart-lang/language#3467
https://github.com/dart-lang/co19/blob/master/LanguageFeatures/Extension-types/dynamic_semantics_member_invocation_A03_t05.dart
https://github.com/dart-lang/co19/blob/master/LanguageFeatures/Extension-types/dynamic_semantics_member_invocation_A03_t06.dart

I'll add more tests

[eernstg]

Thanks! dynamic_semantics_member_invocation_A03_t06.dart gets close because it uses the call of an extension type implicitly in line 45 and 47. But it's an invocation, and the new thing is that it can also be a tear-off.