Generic method instantiation does not apply for the call of function objects (#1616)

Generic function instantiation was never intended to support function objects, it was only intended to be supported for terms that statically resolve to a function declaration. Similarly, generic method instantiation was only intended to be supported for property extractions that statically resolve to a generic method (so the signature was statically known, but the actual implementation was determined at run time).

However, following the wording in the specification, the `call` method of a function type could be used to perform generic method instantiation on an arbitrary function object, which implied that generic function instantiation was supported for arbitrary functions after all.

This PR changes the spec wording such that this (unintended) feature is no more specified to exist.

There is no implementation work in the CFE, because the CFE always reported an error in this case. For the analyzer, see dart-lang/sdk#45936.

Author: eernstg

specification/dartLangSpec.tex

@@ -14545,7 +14545,8 @@ \subsubsection{Generic Method Instantiation}
 Let $i$ be a property extraction expression of the form
 \code{$e$?.\id}, \code{$e$.\id}, or \code{\SUPER.\id}
 (\ref{propertyExtraction}, \ref{superGetterAccessAndMethodClosurization}),
-which is statically resolved to denote an instance method named \id,
+which is statically resolved to denote an instance method named \id{}
+that is not the \CALL{} method of a function type,
 and let $G$ be the static type of $i$.
 Consider the situation where $G$ is a function type of the form
 \RawFunctionType{T_0}{X}{B}{s}{\metavar{parameters}}