Clarify extension member resolution (#4116)

Clarify extension member resolution: The applicability of a given extension for an invocation of an extension instance member `m` would require that the static type of the syntactic receiver does not have 'a member whose basename is the basename of `m`'. This PR makes it explicit that this is 'an instance member whose basename is the basename of `m`', and it isn't relevant if the declaration that induced said type has a static member with such a name.

Author: eernstg

specification/dartLangSpec.tex

@@ -41,6 +41,11 @@
 % version of the language which will actually be specified by the next stable
 % release of this document.
 %
+% Sep 2024
+% - Clarify the extension applicability rule to explicitly state that it
+%   is concerned with an instance member with the same basename, not a
+%   static member.
+%
 % Jun 2024
 % - Add missing references to section 'Type dynamic' at the points where the
 %   static analysis of Object member invocations is specified.
@@ -6661,7 +6666,7 @@ \subsubsection{Applicability of an Extension}
     because they do not have a type.%
   }
 \item
-  The type $S$ does not have a member with the basename $m$,
+  The type $S$ does not have an instance member with the basename $m$,
   and $S$ is neither \DYNAMIC{} nor \code{Never}.
 
   \commentary{%