Reorganized material on null/Null

Author: eernstg

specification/dartLangSpec.tex

@@ -3254,13 +3254,13 @@ \subsubsection{The Operator `=='}
   if the dynamic type of $o$ is a class $C$,
   and $C$ has a primitive operator \lit{==}.
 \item The class \code{Object} has a primitive operator \lit{==}.
+\item The class \code{Null} has a primitive operator \lit{==}.
 \item A class $C$ has a primitive operator \lit{==}
   if it does not have an implementation of the operator \lit{==}
   that overrides the one inherited from \code{Object}.
   \commentary{%
   In particular, the following have a primitive operator \lit{==}:
-  The null object (\ref{null}),
-  function objects obtained by function closurization of
+  Function objects obtained by function closurization of
   a static method or a top-level function
   (\ref{functionClosurization}),
   instances of type \code{bool}
@@ -8669,20 +8669,8 @@ \subsection{Null}
 \end{grammar}
 
 \LMHash{}%
-The null object is the sole instance of the built-in class \code{Null}.
-% The following can be a consequence of the declaration of `Null`,
-% but we don't spell that out, we just require that it is an error.
-Attempting to instantiate \code{Null} causes a compile-time error.
-It is a compile-time error for a class to extend, mix in or implement
-\code{Null}.
-The \code{Null} class extends the \code{Object} class
-and declares no methods except those also declared by \code{Object}.
-In particular, the \code{Null} class does not override the \lit{==} operator
-inherited from the \code{Object} class.
-
-\LMHash{}%
-The static type of \NULL{} is the \code{Null} type.
-
+The static type of \NULL{} is the \code{Null} type
+(\ref{typeNull}).
 
 \subsection{Numbers}
 \LMLabel{numbers}
@@ -21383,7 +21371,7 @@ \subsection{Type \code{Never}}
 
 \LMHash{}%
 The Dart core library \code{dart:core} exports a type named \code{Never}.
-No object has dynamic type \code{Never}.
+No object has a dynamic type which is \code{Never} or a subtype thereof.
 
 \commentary{%
 In other words, \code{Never} is the empty type.
@@ -21410,24 +21398,40 @@ \subsection{Type \code{Null}}
 \LMLabel{typeNull}
 
 \LMHash{}%
-The Dart core library \code{dart:core} exports a type named \code{Null},
+The Dart core library \code{dart:core} exports a type named \code{Null}.
 There is exactly one object whose dynamic type is \code{Null},
-and that is the null object.
+and that is the null object
+(\ref{null}).
 
 \commentary{%
 In other words, \code{Null} is a singleton type.
-\code{Null} is a subtype of all other types
-other than \code{Never} and any type which is
-a subtype of \code{Never}
+Apart from top types
+(\ref{superBoundedTypes}),
+\code{Null} is a subtype of all types of the form \code{$T$?},
+and of all types $S$ such that \futureOrBase{S} is one of the above.
+The only non-trivial subtypes of \code{Null} are
+\code{Never} and subtypes of \code{Never}
 (\ref{subtypeRules}).%
 }
 
+\LMHash{}%
+Attempting to instantiate \code{Null} causes a compile-time error.
+It is a compile-time error for a class to extend, mix in or implement
+\code{Null}.
+The \code{Null} class declares exactly the same members
+with the same signatures as the class \code{Object}.
+
+\commentary{%
+The \code{Null} class has a primitive \lit{==} operator
+(\ref{theOperatorEqualsEquals}).%
+}
+
 \rationale{%
 \code{Null} is mainly used implicitly in
 a type of the form \code{$T$?} for some $T$.
 If $e$ is an expression of type \code{$T$?} that evaluates to an object $o$,
 then $o$ can be an instance of a subtype of $T$,
-or it could be the null object.
+or it can be the null object.
 The latter situation is commonly interpreted to mean that
 the computation did not yield a result.
 In this sense, \code{Null} can be considered to model