Added constraint that the colliding members must be concrete

Author: eernstg

specification/dartLangSpec.tex

@@ -5501,10 +5501,10 @@ \subsection{Mixin Application}
 Let \DefineSymbol{L_M} be the library containing the declaration of $M$.
 
 \LMHash{}%
-Assume that $S$ has a member $m_S$ which is accessible to $L_M$,
+Assume that $S$ has a concrete member $m_S$ which is accessible to $L_M$,
 that $m_S$ has the name $n$ which is private
 (\commentary{so $m_S$ is declared in $L_M$}),
-and that $M$ declares a member $m_M$ which is also named $n$
+and that $M$ declares a concrete member $m_M$ which is also named $n$
 \commentary{(note that $m_M$ is also declared in $L_M$)}.
 In this case a compile-time error occurs.