Specification: Introduce a notion of static invocations

[eernstg]

Cf. dart-lang/sdk#34492, we need the special rule

It is a compile-time error to invoke any of the methods of class 
\code{Object} on a prefix object (\ref{imports})
or on a constant type literal that is immediately followed by
the token `.'\,.

in order to ensure that int.toString() and similar "Object instance method invocations" is a compile-time error. It would be better to specify all static invocations directly and as a separate concept, and then hopefully we could delete this special rule. With that, the expression int.toString() would be a compile-time error because (1) it is a static invocation, so toString can only be a static method, (2) there is no such static method.