Added error messages for when users try to use invalid mixins + a language test that tests that (and tests in each of the front ends).
Removed the tokens from the parser listeners.
Removed all behaviours and error reporting related to these invalid mixins.
Change-Id: I558595826dae7e2c176bd1929e97caa2335c167c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/290614
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Kallen Tu <kallentu@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Kallen Tu
f7bc72fce6