With the experiment flag on, will properly parse the `in`/`out`/`inout`
keywords. Without it, will issue an error regarding the flag. First CL
for parsing, syntax support, no AST work yet.
Keywords only parse correctly for classes and mixins.
See test `variance_disabled_syntax_test.dart`.
Change-Id: I37d765804fa0752512abf52fa44456a173087ef0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/117929
Commit-Queue: Kallen Tu <kallentu@google.com>
Reviewed-by: Leaf Petersen <leafp@google.com>
Reviewed-by: Dan Rubel <danrubel@google.com>
Kallen Tu