Typo fix: mention that L_M != L_C

specification/dartLangSpec.tex

@@ -5508,7 +5508,8 @@ \subsection{Mixin Application}
 Let \DefineSymbol{L_M} be the library containing the declaration of $M$.
 
 \LMHash{}%
-Assume that $S$ has a concrete member $m_S$ which is accessible to $L_M$,
+Assume that $L_C$ and $L_M$ is not the same library,
+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 $n$