Use mixin syntax for Flutter's super-mixins (#22435)

* first pass

* revert WidgetsBindingObserver

* disable prefer_mixin

* docs and more mixins

* newer engine; binding fixes

* upgrade dependencies

* fix test binding

* remove whitespace

* remove the obsolete `enableSuperMixins: true`

* upgrade dartdoc to 0.22.0

* temporarily use git version of dartdoc

* fix pub global activate syntax

* use dartdoc 0.23.0

analysis_options.yaml

@@ -19,8 +19,6 @@
 # Android Studio, and the `flutter analyze` command.
 
 analyzer:
-  language:
-    enableSuperMixins: true
   strong-mode:
     implicit-dynamic: false
   errors:
@@ -134,6 +132,7 @@ linter:
     - prefer_is_empty
     - prefer_is_not_empty
     - prefer_iterable_whereType
+    # - prefer_mixin # https://github.com/dart-lang/language/issues/32
     - prefer_single_quotes
     - prefer_typing_uninitialized_variables
     # - prefer_void_to_null # not yet tested