Add paragraph to make it explicit that "let $S$ be a class" includes the case where it goes via a type alias (#1644)

Before this commit, the section 'Type Aliases' of the language specification specifies that it is an error if a type alias expands to a type variable, and it is used as a mixin. So type aliases expanding to a mixin are already taken into consideration.

However, it is not specified at various usage locations that type aliases are included. They are mentioned in connection with `extends` and `implements` and `with`, but, for instance, not in the section about mixin application.

This commit adds a paragraph where it is stated that it is in general implied that a term `$S$` can be a type alias that transitively expands to a class when we say things like "let `$S$` be a class".

Author: eernstg

specification/dartLangSpec.tex

@@ -20458,6 +20458,15 @@ \subsection{Type Aliases}
 \}
 \end{dartCode}
 
+\LMHash{}%
+When we use phrases like `let $S$ be a class' or `assume that $S$ is a mixin',
+it is understood that this includes the case where
+$S$ is a \synt{typeName} denoting a type alias, or
+$S$ is a parameterized type of the form
+\syntax{<typeName> <typeArguments>}
+where the type name denotes a type alias,
+and the transitive alias expansion of $S$ denotes a class respectively a mixin.
+
 
 \subsection{Subtypes}
 \LMLabel{subtypes}