Update the language spec to allow `with` clause without an `extends` clause
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 <80295e4ef244a07779f93eaa95d582bad8e316a5@google.com>
Reviewed-by: Leaf Petersen <9bef692239b5f624cec8e2cd5249ab1939a377aa@google.com>
Commit-Queue: Dan Rubel <5c5a7a1a3f9344592135bed55605519045cfa711@google.com>

dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk