Update the language spec to allow with clause without an extends clause

This updates the Dart language spec to allow
```
   class C with M
```
where currently the above declaration would require `extends Object`.

Change-Id: Ibb5834b264be7b2c5adb79dfa9a24ad5e1fa539a
Reviewed-on: https://dart-review.googlesource.com/73684
Reviewed-by: Lasse R.H. Nielsen <lrn@google.com>
Reviewed-by: Leaf Petersen <leafp@google.com>
Commit-Queue: Dan Rubel <danrubel@google.com>

docs/language/dartLangSpec.tex

@@ -1234,7 +1234,7 @@ Classes may be defined by class declarations as described below, or via mixin ap
 
 \begin{grammar}
 {\bf classDefinition:}metadata \ABSTRACT{}? \CLASS{} identifier typeParameters?
-  \gnewline{} (superclass mixins?)? interfaces?
+  \gnewline{} superclass? mixins? interfaces?
   \gnewline{} `\{' (metadata classMemberDefinition)* `\}';
   metadata \ABSTRACT{}? \CLASS{} mixinApplicationClass
   .