Correct rule about interfaces of bounded types

Author: eernstg

specification/dartLangSpec.tex

@@ -5588,9 +5588,9 @@ \section{Interfaces}
 (\ref{propertyExtraction}),
 any type $T$ which is $T_0$ bounded
 (\ref{typesBoundedByTypes}),
-where $T_0$ is a class with interface $I$,
+where $T_0$ is a type with interface $I$,
 is also considered to have interface $I$.
-Similarly, when $T$ is $T_0$ bounded where $T_0$ is a function type,
+In particular, when $T$ is $T_0$ bounded where $T_0$ is a function type,
 $T$ is considered to have a method named \CALL{} with signature $m$,
 such that the function type of $m$ is $T_0$.
 
@@ -23713,8 +23713,10 @@ \subsection{Type Nullability}
   For example, even though \code{$e_1$.isEven} is allowed
   when the type of $e_1$ is \code{int},
   \code{$e_2$.isEven} is a compile-time error
-  when the type of $e_2$ is \code{int?}\ or
-  a type variable $X$ whose bound is \code{int?}.%
+  when the type of $e_2$ is \code{int?}.
+  It is also an error for a type variable $X$ whose bound is \code{int?},
+  because they have the same interface
+  (\ref{interfaces}).%
 }